locking/percpu-rwsem: Optimize readers and reduce global impact
Currently the percpu-rwsem switches to (global) atomic ops while a writer is waiting; which could be quite a while and slows down releasing the readers. This patch cures this problem by ordering the reader-state vs reader-count (see the comments in __percpu_down_read() and percpu_down_write()). This changes a global atomic op into a full memory barrier, which doesn't have the global cacheline contention. This also enables using the percpu-rwsem with rcu_sync disabled in order to bias the implementation differently, reducing the writer latency by adding some cost to readers. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org [ Fixed modular build. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
Ingo Molnar
parent
08be8f63c4
commit
80127a3968
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@ -10,30 +10,96 @@
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struct percpu_rw_semaphore {
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struct rcu_sync rss;
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unsigned int __percpu *fast_read_ctr;
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unsigned int __percpu *read_count;
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struct rw_semaphore rw_sem;
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atomic_t slow_read_ctr;
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wait_queue_head_t write_waitq;
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wait_queue_head_t writer;
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int readers_block;
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};
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extern void percpu_down_read(struct percpu_rw_semaphore *);
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extern int percpu_down_read_trylock(struct percpu_rw_semaphore *);
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extern void percpu_up_read(struct percpu_rw_semaphore *);
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extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
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extern void __percpu_up_read(struct percpu_rw_semaphore *);
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static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
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{
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might_sleep();
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rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
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preempt_disable();
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/*
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* We are in an RCU-sched read-side critical section, so the writer
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* cannot both change sem->state from readers_fast and start checking
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* counters while we are here. So if we see !sem->state, we know that
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* the writer won't be checking until we're past the preempt_enable()
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* and that one the synchronize_sched() is done, the writer will see
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* anything we did within this RCU-sched read-size critical section.
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*/
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__this_cpu_inc(*sem->read_count);
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if (unlikely(!rcu_sync_is_idle(&sem->rss)))
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__percpu_down_read(sem, false); /* Unconditional memory barrier */
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preempt_enable();
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/*
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* The barrier() from preempt_enable() prevents the compiler from
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* bleeding the critical section out.
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*/
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}
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static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
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{
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int ret = 1;
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preempt_disable();
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/*
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* Same as in percpu_down_read().
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*/
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__this_cpu_inc(*sem->read_count);
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if (unlikely(!rcu_sync_is_idle(&sem->rss)))
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ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
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preempt_enable();
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/*
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* The barrier() from preempt_enable() prevents the compiler from
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* bleeding the critical section out.
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*/
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if (ret)
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rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);
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return ret;
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}
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static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
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{
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/*
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* The barrier() in preempt_disable() prevents the compiler from
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* bleeding the critical section out.
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*/
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preempt_disable();
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/*
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* Same as in percpu_down_read().
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*/
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if (likely(rcu_sync_is_idle(&sem->rss)))
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__this_cpu_dec(*sem->read_count);
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else
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__percpu_up_read(sem); /* Unconditional memory barrier */
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preempt_enable();
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rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
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}
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extern void percpu_down_write(struct percpu_rw_semaphore *);
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extern void percpu_up_write(struct percpu_rw_semaphore *);
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extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
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const char *, struct lock_class_key *);
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extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
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#define percpu_init_rwsem(brw) \
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#define percpu_init_rwsem(sem) \
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({ \
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static struct lock_class_key rwsem_key; \
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__percpu_init_rwsem(brw, #brw, &rwsem_key); \
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__percpu_init_rwsem(sem, #sem, &rwsem_key); \
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})
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#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)
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static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
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@ -8,152 +8,186 @@
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#include <linux/sched.h>
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#include <linux/errno.h>
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int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
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int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
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const char *name, struct lock_class_key *rwsem_key)
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{
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brw->fast_read_ctr = alloc_percpu(int);
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if (unlikely(!brw->fast_read_ctr))
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sem->read_count = alloc_percpu(int);
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if (unlikely(!sem->read_count))
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return -ENOMEM;
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/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
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__init_rwsem(&brw->rw_sem, name, rwsem_key);
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rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
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atomic_set(&brw->slow_read_ctr, 0);
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init_waitqueue_head(&brw->write_waitq);
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rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
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__init_rwsem(&sem->rw_sem, name, rwsem_key);
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init_waitqueue_head(&sem->writer);
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sem->readers_block = 0;
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return 0;
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}
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EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
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void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
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void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
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{
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/*
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* XXX: temporary kludge. The error path in alloc_super()
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* assumes that percpu_free_rwsem() is safe after kzalloc().
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*/
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if (!brw->fast_read_ctr)
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if (!sem->read_count)
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return;
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rcu_sync_dtor(&brw->rss);
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free_percpu(brw->fast_read_ctr);
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brw->fast_read_ctr = NULL; /* catch use after free bugs */
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rcu_sync_dtor(&sem->rss);
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free_percpu(sem->read_count);
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sem->read_count = NULL; /* catch use after free bugs */
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}
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EXPORT_SYMBOL_GPL(percpu_free_rwsem);
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/*
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* This is the fast-path for down_read/up_read. If it succeeds we rely
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* on the barriers provided by rcu_sync_enter/exit; see the comments in
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* percpu_down_write() and percpu_up_write().
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*
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* If this helper fails the callers rely on the normal rw_semaphore and
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* atomic_dec_and_test(), so in this case we have the necessary barriers.
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*/
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static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
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{
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bool success;
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preempt_disable();
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success = rcu_sync_is_idle(&brw->rss);
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if (likely(success))
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__this_cpu_add(*brw->fast_read_ctr, val);
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preempt_enable();
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return success;
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}
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/*
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* Like the normal down_read() this is not recursive, the writer can
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* come after the first percpu_down_read() and create the deadlock.
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*
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* Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
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* percpu_up_read() does rwsem_release(). This pairs with the usage
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* of ->rw_sem in percpu_down/up_write().
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*/
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void percpu_down_read(struct percpu_rw_semaphore *brw)
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{
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might_sleep();
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rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
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if (likely(update_fast_ctr(brw, +1)))
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return;
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/* Avoid rwsem_acquire_read() and rwsem_release() */
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__down_read(&brw->rw_sem);
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atomic_inc(&brw->slow_read_ctr);
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__up_read(&brw->rw_sem);
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}
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EXPORT_SYMBOL_GPL(percpu_down_read);
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int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
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{
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if (unlikely(!update_fast_ctr(brw, +1))) {
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if (!__down_read_trylock(&brw->rw_sem))
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return 0;
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atomic_inc(&brw->slow_read_ctr);
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__up_read(&brw->rw_sem);
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}
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rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
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return 1;
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}
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void percpu_up_read(struct percpu_rw_semaphore *brw)
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{
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rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
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if (likely(update_fast_ctr(brw, -1)))
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return;
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/* false-positive is possible but harmless */
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if (atomic_dec_and_test(&brw->slow_read_ctr))
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wake_up_all(&brw->write_waitq);
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}
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EXPORT_SYMBOL_GPL(percpu_up_read);
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static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
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{
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unsigned int sum = 0;
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int cpu;
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for_each_possible_cpu(cpu) {
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sum += per_cpu(*brw->fast_read_ctr, cpu);
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per_cpu(*brw->fast_read_ctr, cpu) = 0;
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}
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return sum;
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}
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void percpu_down_write(struct percpu_rw_semaphore *brw)
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int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
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{
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/*
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* Make rcu_sync_is_idle() == F and thus disable the fast-path in
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* percpu_down_read() and percpu_up_read(), and wait for gp pass.
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* Due to having preemption disabled the decrement happens on
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* the same CPU as the increment, avoiding the
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* increment-on-one-CPU-and-decrement-on-another problem.
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*
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* The latter synchronises us with the preceding readers which used
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* the fast-past, so we can not miss the result of __this_cpu_add()
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* or anything else inside their criticial sections.
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* If the reader misses the writer's assignment of readers_block, then
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* the writer is guaranteed to see the reader's increment.
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*
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* Conversely, any readers that increment their sem->read_count after
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* the writer looks are guaranteed to see the readers_block value,
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* which in turn means that they are guaranteed to immediately
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* decrement their sem->read_count, so that it doesn't matter that the
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* writer missed them.
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*/
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rcu_sync_enter(&brw->rss);
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/* exclude other writers, and block the new readers completely */
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down_write(&brw->rw_sem);
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smp_mb(); /* A matches D */
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/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
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atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
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/*
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* If !readers_block the critical section starts here, matched by the
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* release in percpu_up_write().
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*/
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if (likely(!smp_load_acquire(&sem->readers_block)))
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return 1;
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/* wait for all readers to complete their percpu_up_read() */
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wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
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/*
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* Per the above comment; we still have preemption disabled and
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* will thus decrement on the same CPU as we incremented.
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*/
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__percpu_up_read(sem);
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if (try)
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return 0;
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/*
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* We either call schedule() in the wait, or we'll fall through
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* and reschedule on the preempt_enable() in percpu_down_read().
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*/
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preempt_enable_no_resched();
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/*
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* Avoid lockdep for the down/up_read() we already have them.
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*/
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__down_read(&sem->rw_sem);
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this_cpu_inc(*sem->read_count);
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__up_read(&sem->rw_sem);
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preempt_disable();
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return 1;
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}
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EXPORT_SYMBOL_GPL(__percpu_down_read);
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void __percpu_up_read(struct percpu_rw_semaphore *sem)
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{
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smp_mb(); /* B matches C */
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/*
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* In other words, if they see our decrement (presumably to aggregate
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* zero, as that is the only time it matters) they will also see our
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* critical section.
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*/
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__this_cpu_dec(*sem->read_count);
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/* Prod writer to recheck readers_active */
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wake_up(&sem->writer);
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}
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EXPORT_SYMBOL_GPL(__percpu_up_read);
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#define per_cpu_sum(var) \
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({ \
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typeof(var) __sum = 0; \
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int cpu; \
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compiletime_assert_atomic_type(__sum); \
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for_each_possible_cpu(cpu) \
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__sum += per_cpu(var, cpu); \
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__sum; \
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})
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/*
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* Return true if the modular sum of the sem->read_count per-CPU variable is
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* zero. If this sum is zero, then it is stable due to the fact that if any
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* newly arriving readers increment a given counter, they will immediately
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* decrement that same counter.
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*/
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static bool readers_active_check(struct percpu_rw_semaphore *sem)
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{
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if (per_cpu_sum(*sem->read_count) != 0)
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return false;
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/*
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* If we observed the decrement; ensure we see the entire critical
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* section.
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*/
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smp_mb(); /* C matches B */
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return true;
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}
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void percpu_down_write(struct percpu_rw_semaphore *sem)
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{
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/* Notify readers to take the slow path. */
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rcu_sync_enter(&sem->rss);
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down_write(&sem->rw_sem);
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/*
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* Notify new readers to block; up until now, and thus throughout the
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* longish rcu_sync_enter() above, new readers could still come in.
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*/
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WRITE_ONCE(sem->readers_block, 1);
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smp_mb(); /* D matches A */
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/*
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* If they don't see our writer of readers_block, then we are
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* guaranteed to see their sem->read_count increment, and therefore
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* will wait for them.
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*/
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/* Wait for all now active readers to complete. */
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wait_event(sem->writer, readers_active_check(sem));
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}
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EXPORT_SYMBOL_GPL(percpu_down_write);
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void percpu_up_write(struct percpu_rw_semaphore *brw)
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void percpu_up_write(struct percpu_rw_semaphore *sem)
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{
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/* release the lock, but the readers can't use the fast-path */
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up_write(&brw->rw_sem);
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/*
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* Enable the fast-path in percpu_down_read() and percpu_up_read()
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* but only after another gp pass; this adds the necessary barrier
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* to ensure the reader can't miss the changes done by us.
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* Signal the writer is done, no fast path yet.
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*
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* One reason that we cannot just immediately flip to readers_fast is
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* that new readers might fail to see the results of this writer's
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* critical section.
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*
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* Therefore we force it through the slow path which guarantees an
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* acquire and thereby guarantees the critical section's consistency.
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*/
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rcu_sync_exit(&brw->rss);
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smp_store_release(&sem->readers_block, 0);
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/*
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* Release the write lock, this will allow readers back in the game.
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*/
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up_write(&sem->rw_sem);
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/*
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* Once this completes (at least one RCU-sched grace period hence) the
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* reader fast path will be available again. Safe to use outside the
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* exclusive write lock because its counting.
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*/
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rcu_sync_exit(&sem->rss);
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}
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EXPORT_SYMBOL_GPL(percpu_up_write);
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@ -68,6 +68,8 @@ void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
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RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
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"suspicious rcu_sync_is_idle() usage");
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}
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EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
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#endif
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/**
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