602 lines
16 KiB
C
602 lines
16 KiB
C
#ifndef __LINUX_SEQLOCK_H
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#define __LINUX_SEQLOCK_H
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/*
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* Reader/writer consistent mechanism without starving writers. This type of
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* lock for data where the reader wants a consistent set of information
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* and is willing to retry if the information changes. There are two types
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* of readers:
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* 1. Sequence readers which never block a writer but they may have to retry
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* if a writer is in progress by detecting change in sequence number.
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* Writers do not wait for a sequence reader.
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* 2. Locking readers which will wait if a writer or another locking reader
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* is in progress. A locking reader in progress will also block a writer
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* from going forward. Unlike the regular rwlock, the read lock here is
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* exclusive so that only one locking reader can get it.
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*
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* This is not as cache friendly as brlock. Also, this may not work well
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* for data that contains pointers, because any writer could
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* invalidate a pointer that a reader was following.
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*
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* Expected non-blocking reader usage:
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* do {
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* seq = read_seqbegin(&foo);
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* ...
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* } while (read_seqretry(&foo, seq));
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*
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*
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* On non-SMP the spin locks disappear but the writer still needs
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* to increment the sequence variables because an interrupt routine could
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* change the state of the data.
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*
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* Based on x86_64 vsyscall gettimeofday
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* by Keith Owens and Andrea Arcangeli
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*/
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#include <linux/spinlock.h>
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#include <linux/preempt.h>
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#include <linux/lockdep.h>
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#include <linux/compiler.h>
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#include <asm/processor.h>
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/*
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* Version using sequence counter only.
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* This can be used when code has its own mutex protecting the
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* updating starting before the write_seqcountbeqin() and ending
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* after the write_seqcount_end().
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*/
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typedef struct seqcount {
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unsigned sequence;
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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struct lockdep_map dep_map;
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#endif
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} seqcount_t;
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static inline void __seqcount_init(seqcount_t *s, const char *name,
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struct lock_class_key *key)
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{
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/*
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* Make sure we are not reinitializing a held lock:
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*/
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lockdep_init_map(&s->dep_map, name, key, 0);
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s->sequence = 0;
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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# define SEQCOUNT_DEP_MAP_INIT(lockname) \
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.dep_map = { .name = #lockname } \
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# define seqcount_init(s) \
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do { \
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static struct lock_class_key __key; \
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__seqcount_init((s), #s, &__key); \
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} while (0)
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static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
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{
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seqcount_t *l = (seqcount_t *)s;
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unsigned long flags;
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local_irq_save(flags);
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seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
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seqcount_release(&l->dep_map, 1, _RET_IP_);
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local_irq_restore(flags);
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}
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#else
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# define SEQCOUNT_DEP_MAP_INIT(lockname)
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# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
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# define seqcount_lockdep_reader_access(x)
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#endif
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#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
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/**
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* __read_seqcount_begin - begin a seq-read critical section (without barrier)
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
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* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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* provided before actually loading any of the variables that are to be
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* protected in this critical section.
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*
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* Use carefully, only in critical code, and comment how the barrier is
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* provided.
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*/
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static inline unsigned __read_seqcount_begin(const seqcount_t *s)
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{
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unsigned ret;
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repeat:
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ret = READ_ONCE(s->sequence);
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if (unlikely(ret & 1)) {
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cpu_relax();
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goto repeat;
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}
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return ret;
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}
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/**
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* raw_read_seqcount - Read the raw seqcount
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* raw_read_seqcount opens a read critical section of the given
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* seqcount without any lockdep checking and without checking or
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* masking the LSB. Calling code is responsible for handling that.
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*/
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static inline unsigned raw_read_seqcount(const seqcount_t *s)
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{
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unsigned ret = READ_ONCE(s->sequence);
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smp_rmb();
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return ret;
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}
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/**
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* raw_read_seqcount_begin - start seq-read critical section w/o lockdep
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* raw_read_seqcount_begin opens a read critical section of the given
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* seqcount, but without any lockdep checking. Validity of the critical
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* section is tested by checking read_seqcount_retry function.
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*/
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static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
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{
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unsigned ret = __read_seqcount_begin(s);
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smp_rmb();
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return ret;
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}
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/**
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* read_seqcount_begin - begin a seq-read critical section
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* read_seqcount_begin opens a read critical section of the given seqcount.
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* Validity of the critical section is tested by checking read_seqcount_retry
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* function.
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*/
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static inline unsigned read_seqcount_begin(const seqcount_t *s)
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{
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seqcount_lockdep_reader_access(s);
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return raw_read_seqcount_begin(s);
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}
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/**
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* raw_seqcount_begin - begin a seq-read critical section
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* raw_seqcount_begin opens a read critical section of the given seqcount.
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* Validity of the critical section is tested by checking read_seqcount_retry
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* function.
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*
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* Unlike read_seqcount_begin(), this function will not wait for the count
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* to stabilize. If a writer is active when we begin, we will fail the
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* read_seqcount_retry() instead of stabilizing at the beginning of the
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* critical section.
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*/
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static inline unsigned raw_seqcount_begin(const seqcount_t *s)
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{
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unsigned ret = READ_ONCE(s->sequence);
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smp_rmb();
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return ret & ~1;
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}
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/**
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* __read_seqcount_retry - end a seq-read critical section (without barrier)
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* @s: pointer to seqcount_t
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* @start: count, from read_seqcount_begin
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* Returns: 1 if retry is required, else 0
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*
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* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
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* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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* provided before actually loading any of the variables that are to be
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* protected in this critical section.
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*
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* Use carefully, only in critical code, and comment how the barrier is
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* provided.
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*/
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static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
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{
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return unlikely(s->sequence != start);
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}
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/**
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* read_seqcount_retry - end a seq-read critical section
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* @s: pointer to seqcount_t
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* @start: count, from read_seqcount_begin
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* Returns: 1 if retry is required, else 0
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*
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* read_seqcount_retry closes a read critical section of the given seqcount.
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* If the critical section was invalid, it must be ignored (and typically
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* retried).
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*/
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static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
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{
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smp_rmb();
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return __read_seqcount_retry(s, start);
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}
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static inline void raw_write_seqcount_begin(seqcount_t *s)
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{
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s->sequence++;
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smp_wmb();
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}
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static inline void raw_write_seqcount_end(seqcount_t *s)
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{
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smp_wmb();
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s->sequence++;
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}
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/**
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* raw_write_seqcount_barrier - do a seq write barrier
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* @s: pointer to seqcount_t
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*
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* This can be used to provide an ordering guarantee instead of the
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* usual consistency guarantee. It is one wmb cheaper, because we can
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* collapse the two back-to-back wmb()s.
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*
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* seqcount_t seq;
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* bool X = true, Y = false;
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*
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* void read(void)
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* {
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* bool x, y;
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*
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* do {
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* int s = read_seqcount_begin(&seq);
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*
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* x = X; y = Y;
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*
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* } while (read_seqcount_retry(&seq, s));
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*
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* BUG_ON(!x && !y);
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* }
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*
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* void write(void)
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* {
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* Y = true;
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*
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* raw_write_seqcount_barrier(seq);
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*
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* X = false;
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* }
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*/
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static inline void raw_write_seqcount_barrier(seqcount_t *s)
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{
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s->sequence++;
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smp_wmb();
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s->sequence++;
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}
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static inline int raw_read_seqcount_latch(seqcount_t *s)
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{
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int seq = READ_ONCE(s->sequence);
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/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
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smp_read_barrier_depends();
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return seq;
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}
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/**
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* raw_write_seqcount_latch - redirect readers to even/odd copy
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* @s: pointer to seqcount_t
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*
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* The latch technique is a multiversion concurrency control method that allows
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* queries during non-atomic modifications. If you can guarantee queries never
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* interrupt the modification -- e.g. the concurrency is strictly between CPUs
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* -- you most likely do not need this.
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*
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* Where the traditional RCU/lockless data structures rely on atomic
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* modifications to ensure queries observe either the old or the new state the
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* latch allows the same for non-atomic updates. The trade-off is doubling the
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* cost of storage; we have to maintain two copies of the entire data
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* structure.
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*
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* Very simply put: we first modify one copy and then the other. This ensures
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* there is always one copy in a stable state, ready to give us an answer.
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*
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* The basic form is a data structure like:
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*
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* struct latch_struct {
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* seqcount_t seq;
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* struct data_struct data[2];
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* };
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*
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* Where a modification, which is assumed to be externally serialized, does the
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* following:
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*
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* void latch_modify(struct latch_struct *latch, ...)
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* {
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* smp_wmb(); <- Ensure that the last data[1] update is visible
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* latch->seq++;
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* smp_wmb(); <- Ensure that the seqcount update is visible
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*
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* modify(latch->data[0], ...);
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*
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* smp_wmb(); <- Ensure that the data[0] update is visible
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* latch->seq++;
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* smp_wmb(); <- Ensure that the seqcount update is visible
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*
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* modify(latch->data[1], ...);
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* }
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*
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* The query will have a form like:
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*
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* struct entry *latch_query(struct latch_struct *latch, ...)
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* {
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* struct entry *entry;
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* unsigned seq, idx;
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*
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* do {
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* seq = raw_read_seqcount_latch(&latch->seq);
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*
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* idx = seq & 0x01;
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* entry = data_query(latch->data[idx], ...);
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*
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* smp_rmb();
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* } while (seq != latch->seq);
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*
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* return entry;
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* }
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*
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* So during the modification, queries are first redirected to data[1]. Then we
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* modify data[0]. When that is complete, we redirect queries back to data[0]
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* and we can modify data[1].
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*
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* NOTE: The non-requirement for atomic modifications does _NOT_ include
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* the publishing of new entries in the case where data is a dynamic
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* data structure.
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*
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* An iteration might start in data[0] and get suspended long enough
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* to miss an entire modification sequence, once it resumes it might
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* observe the new entry.
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*
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* NOTE: When data is a dynamic data structure; one should use regular RCU
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* patterns to manage the lifetimes of the objects within.
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*/
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static inline void raw_write_seqcount_latch(seqcount_t *s)
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{
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smp_wmb(); /* prior stores before incrementing "sequence" */
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s->sequence++;
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smp_wmb(); /* increment "sequence" before following stores */
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}
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/*
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* Sequence counter only version assumes that callers are using their
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* own mutexing.
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*/
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static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
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{
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raw_write_seqcount_begin(s);
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seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
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}
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static inline void write_seqcount_begin(seqcount_t *s)
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{
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write_seqcount_begin_nested(s, 0);
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}
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static inline void write_seqcount_end(seqcount_t *s)
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{
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seqcount_release(&s->dep_map, 1, _RET_IP_);
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raw_write_seqcount_end(s);
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}
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/**
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* write_seqcount_invalidate - invalidate in-progress read-side seq operations
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* @s: pointer to seqcount_t
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*
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* After write_seqcount_invalidate, no read-side seq operations will complete
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* successfully and see data older than this.
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*/
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static inline void write_seqcount_invalidate(seqcount_t *s)
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{
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smp_wmb();
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s->sequence+=2;
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}
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typedef struct {
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struct seqcount seqcount;
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spinlock_t lock;
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} seqlock_t;
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/*
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* These macros triggered gcc-3.x compile-time problems. We think these are
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* OK now. Be cautious.
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*/
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#define __SEQLOCK_UNLOCKED(lockname) \
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{ \
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.seqcount = SEQCNT_ZERO(lockname), \
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.lock = __SPIN_LOCK_UNLOCKED(lockname) \
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}
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#define seqlock_init(x) \
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do { \
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seqcount_init(&(x)->seqcount); \
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spin_lock_init(&(x)->lock); \
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} while (0)
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#define DEFINE_SEQLOCK(x) \
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seqlock_t x = __SEQLOCK_UNLOCKED(x)
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/*
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* Read side functions for starting and finalizing a read side section.
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*/
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static inline unsigned read_seqbegin(const seqlock_t *sl)
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{
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return read_seqcount_begin(&sl->seqcount);
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}
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static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
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{
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return read_seqcount_retry(&sl->seqcount, start);
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}
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/*
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* Lock out other writers and update the count.
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* Acts like a normal spin_lock/unlock.
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* Don't need preempt_disable() because that is in the spin_lock already.
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*/
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static inline void write_seqlock(seqlock_t *sl)
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{
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spin_lock(&sl->lock);
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write_seqcount_begin(&sl->seqcount);
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}
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static inline void write_sequnlock(seqlock_t *sl)
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{
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write_seqcount_end(&sl->seqcount);
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spin_unlock(&sl->lock);
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}
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static inline void write_seqlock_bh(seqlock_t *sl)
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{
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spin_lock_bh(&sl->lock);
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write_seqcount_begin(&sl->seqcount);
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}
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static inline void write_sequnlock_bh(seqlock_t *sl)
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{
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write_seqcount_end(&sl->seqcount);
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spin_unlock_bh(&sl->lock);
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}
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static inline void write_seqlock_irq(seqlock_t *sl)
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{
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spin_lock_irq(&sl->lock);
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write_seqcount_begin(&sl->seqcount);
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}
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static inline void write_sequnlock_irq(seqlock_t *sl)
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{
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write_seqcount_end(&sl->seqcount);
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spin_unlock_irq(&sl->lock);
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}
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static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
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{
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unsigned long flags;
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spin_lock_irqsave(&sl->lock, flags);
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write_seqcount_begin(&sl->seqcount);
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return flags;
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}
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#define write_seqlock_irqsave(lock, flags) \
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do { flags = __write_seqlock_irqsave(lock); } while (0)
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static inline void
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write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
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{
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write_seqcount_end(&sl->seqcount);
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spin_unlock_irqrestore(&sl->lock, flags);
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}
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/*
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* A locking reader exclusively locks out other writers and locking readers,
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* but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
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* Don't need preempt_disable() because that is in the spin_lock already.
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*/
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static inline void read_seqlock_excl(seqlock_t *sl)
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{
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spin_lock(&sl->lock);
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}
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static inline void read_sequnlock_excl(seqlock_t *sl)
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{
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spin_unlock(&sl->lock);
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}
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/**
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* read_seqbegin_or_lock - begin a sequence number check or locking block
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* @lock: sequence lock
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* @seq : sequence number to be checked
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*
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* First try it once optimistically without taking the lock. If that fails,
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* take the lock. The sequence number is also used as a marker for deciding
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* whether to be a reader (even) or writer (odd).
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* N.B. seq must be initialized to an even number to begin with.
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*/
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static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
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{
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if (!(*seq & 1)) /* Even */
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*seq = read_seqbegin(lock);
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else /* Odd */
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read_seqlock_excl(lock);
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}
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static inline int need_seqretry(seqlock_t *lock, int seq)
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{
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return !(seq & 1) && read_seqretry(lock, seq);
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}
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static inline void done_seqretry(seqlock_t *lock, int seq)
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{
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if (seq & 1)
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read_sequnlock_excl(lock);
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}
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|
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static inline void read_seqlock_excl_bh(seqlock_t *sl)
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{
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spin_lock_bh(&sl->lock);
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}
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|
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static inline void read_sequnlock_excl_bh(seqlock_t *sl)
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{
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|
spin_unlock_bh(&sl->lock);
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|
}
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static inline void read_seqlock_excl_irq(seqlock_t *sl)
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|
{
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spin_lock_irq(&sl->lock);
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|
}
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|
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static inline void read_sequnlock_excl_irq(seqlock_t *sl)
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|
{
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|
spin_unlock_irq(&sl->lock);
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|
}
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|
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static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
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|
{
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|
unsigned long flags;
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|
|
|
spin_lock_irqsave(&sl->lock, flags);
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|
return flags;
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|
}
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|
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#define read_seqlock_excl_irqsave(lock, flags) \
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do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
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|
|
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static inline void
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read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
|
|
{
|
|
spin_unlock_irqrestore(&sl->lock, flags);
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|
}
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|
|
|
static inline unsigned long
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|
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
|
|
{
|
|
unsigned long flags = 0;
|
|
|
|
if (!(*seq & 1)) /* Even */
|
|
*seq = read_seqbegin(lock);
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|
else /* Odd */
|
|
read_seqlock_excl_irqsave(lock, flags);
|
|
|
|
return flags;
|
|
}
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|
|
|
static inline void
|
|
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
|
|
{
|
|
if (seq & 1)
|
|
read_sequnlock_excl_irqrestore(lock, flags);
|
|
}
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|
#endif /* __LINUX_SEQLOCK_H */
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