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locking/rt: Add base code for RT rw_semaphore and rwlock 

On PREEMPT_RT, rw_semaphores and rwlocks are substituted with an rtmutex and
a reader count. The implementation is writer unfair, as it is not feasible
to do priority inheritance on multiple readers, but experience has shown
that real-time workloads are not the typical workloads which are sensitive
to writer starvation.

The inner workings of rw_semaphores and rwlocks on RT are almost identical
except for the task state and signal handling. rw_semaphores are not state
preserving over a contention, they are expected to enter and leave with state
== TASK_RUNNING. rwlocks have a mechanism to preserve the state of the task
at entry and restore it after unblocking taking potential non-lock related
wakeups into account. rw_semaphores can also be subject to signal handling
interrupting a blocked state, while rwlocks ignore signals.

To avoid code duplication, provide a shared implementation which takes the
small difference vs. state and signals into account. The code is included
into the relevant rw_semaphore/rwlock base code and compiled for each use
case separately.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211302.957920571@linutronix.de
This commit is contained in:
Thomas Gleixner 2021-08-15 23:28:03 +02:00 committed by Ingo Molnar
parent 6bc8996add
commit 943f0edb75
2 changed files with 302 additions and 0 deletions
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39
include/linux/rwbase_rt.h Normal file
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// SPDX-License-Identifier: GPL-2.0-only
#ifndef _LINUX_RWBASE_RT_H
#define _LINUX_RWBASE_RT_H
#include <linux/rtmutex.h>
#include <linux/atomic.h>
#define READER_BIAS (1U << 31)
#define WRITER_BIAS (1U << 30)
struct rwbase_rt {
atomic_t readers;
struct rt_mutex_base rtmutex;
};
#define __RWBASE_INITIALIZER(name) \
{ \
.readers = ATOMIC_INIT(READER_BIAS), \
.rtmutex = __RT_MUTEX_BASE_INITIALIZER(name.rtmutex), \
}
#define init_rwbase_rt(rwbase) \
do { \
rt_mutex_base_init(&(rwbase)->rtmutex); \
atomic_set(&(rwbase)->readers, READER_BIAS); \
} while (0)
static __always_inline bool rw_base_is_locked(struct rwbase_rt *rwb)
{
return atomic_read(&rwb->readers) != READER_BIAS;
}
static __always_inline bool rw_base_is_contended(struct rwbase_rt *rwb)
{
return atomic_read(&rwb->readers) > 0;
}
#endif /* _LINUX_RWBASE_RT_H */

263
kernel/locking/rwbase_rt.c Normal file
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// SPDX-License-Identifier: GPL-2.0-only
/*
* RT-specific reader/writer semaphores and reader/writer locks
*
* down_write/write_lock()
* 1) Lock rtmutex
* 2) Remove the reader BIAS to force readers into the slow path
* 3) Wait until all readers have left the critical section
* 4) Mark it write locked
*
* up_write/write_unlock()
* 1) Remove the write locked marker
* 2) Set the reader BIAS, so readers can use the fast path again
* 3) Unlock rtmutex, to release blocked readers
*
* down_read/read_lock()
* 1) Try fast path acquisition (reader BIAS is set)
* 2) Take tmutex::wait_lock, which protects the writelocked flag
* 3) If !writelocked, acquire it for read
* 4) If writelocked, block on tmutex
* 5) unlock rtmutex, goto 1)
*
* up_read/read_unlock()
* 1) Try fast path release (reader count != 1)
* 2) Wake the writer waiting in down_write()/write_lock() #3
*
* down_read/read_lock()#3 has the consequence, that rw semaphores and rw
* locks on RT are not writer fair, but writers, which should be avoided in
* RT tasks (think mmap_sem), are subject to the rtmutex priority/DL
* inheritance mechanism.
*
* It's possible to make the rw primitives writer fair by keeping a list of
* active readers. A blocked writer would force all newly incoming readers
* to block on the rtmutex, but the rtmutex would have to be proxy locked
* for one reader after the other. We can't use multi-reader inheritance
* because there is no way to support that with SCHED_DEADLINE.
* Implementing the one by one reader boosting/handover mechanism is a
* major surgery for a very dubious value.
*
* The risk of writer starvation is there, but the pathological use cases
* which trigger it are not necessarily the typical RT workloads.
*
* Common code shared between RT rw_semaphore and rwlock
*/
static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
{
int r;
/*
* Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
* set.
*/
for (r = atomic_read(&rwb->readers); r < 0;) {
if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))
return 1;
}
return 0;
}
static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
unsigned int state)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
int ret;
raw_spin_lock_irq(&rtm->wait_lock);
/*
* Allow readers, as long as the writer has not completely
* acquired the semaphore for write.
*/
if (atomic_read(&rwb->readers) != WRITER_BIAS) {
atomic_inc(&rwb->readers);
raw_spin_unlock_irq(&rtm->wait_lock);
return 0;
}
/*
* Call into the slow lock path with the rtmutex->wait_lock
* held, so this can't result in the following race:
*
* Reader1 Reader2 Writer
* down_read()
* down_write()
* rtmutex_lock(m)
* wait()
* down_read()
* unlock(m->wait_lock)
* up_read()
* wake(Writer)
* lock(m->wait_lock)
* sem->writelocked=true
* unlock(m->wait_lock)
*
* up_write()
* sem->writelocked=false
* rtmutex_unlock(m)
* down_read()
* down_write()
* rtmutex_lock(m)
* wait()
* rtmutex_lock(m)
*
* That would put Reader1 behind the writer waiting on
* Reader2 to call up_read(), which might be unbound.
*/
/*
* For rwlocks this returns 0 unconditionally, so the below
* !ret conditionals are optimized out.
*/
ret = rwbase_rtmutex_slowlock_locked(rtm, state);
/*
* On success the rtmutex is held, so there can't be a writer
* active. Increment the reader count and immediately drop the
* rtmutex again.
*
* rtmutex->wait_lock has to be unlocked in any case of course.
*/
if (!ret)
atomic_inc(&rwb->readers);
raw_spin_unlock_irq(&rtm->wait_lock);
if (!ret)
rwbase_rtmutex_unlock(rtm);
return ret;
}
static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
unsigned int state)
{
if (rwbase_read_trylock(rwb))
return 0;
return __rwbase_read_lock(rwb, state);
}
static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
unsigned int state)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
struct task_struct *owner;
raw_spin_lock_irq(&rtm->wait_lock);
/*
* Wake the writer, i.e. the rtmutex owner. It might release the
* rtmutex concurrently in the fast path (due to a signal), but to
* clean up rwb->readers it needs to acquire rtm->wait_lock. The
* worst case which can happen is a spurious wakeup.
*/
owner = rt_mutex_owner(rtm);
if (owner)
wake_up_state(owner, state);
raw_spin_unlock_irq(&rtm->wait_lock);
}
static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
unsigned int state)
{
/*
* rwb->readers can only hit 0 when a writer is waiting for the
* active readers to leave the critical section.
*/
if (unlikely(atomic_dec_and_test(&rwb->readers)))
__rwbase_read_unlock(rwb, state);
}
static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
unsigned long flags)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
atomic_add(READER_BIAS - bias, &rwb->readers);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
rwbase_rtmutex_unlock(rtm);
}
static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
unsigned long flags;
raw_spin_lock_irqsave(&rtm->wait_lock, flags);
__rwbase_write_unlock(rwb, WRITER_BIAS, flags);
}
static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
unsigned long flags;
raw_spin_lock_irqsave(&rtm->wait_lock, flags);
/* Release it and account current as reader */
__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
}
static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
unsigned int state)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
unsigned long flags;
/* Take the rtmutex as a first step */
if (rwbase_rtmutex_lock_state(rtm, state))
return -EINTR;
/* Force readers into slow path */
atomic_sub(READER_BIAS, &rwb->readers);
raw_spin_lock_irqsave(&rtm->wait_lock, flags);
/*
* set_current_state() for rw_semaphore
* current_save_and_set_rtlock_wait_state() for rwlock
*/
rwbase_set_and_save_current_state(state);
/* Block until all readers have left the critical section. */
for (; atomic_read(&rwb->readers);) {
/* Optimized out for rwlocks */
if (rwbase_signal_pending_state(state, current)) {
__set_current_state(TASK_RUNNING);
__rwbase_write_unlock(rwb, 0, flags);
return -EINTR;
}
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
/*
* Schedule and wait for the readers to leave the critical
* section. The last reader leaving it wakes the waiter.
*/
if (atomic_read(&rwb->readers) != 0)
rwbase_schedule();
set_current_state(state);
raw_spin_lock_irqsave(&rtm->wait_lock, flags);
}
atomic_set(&rwb->readers, WRITER_BIAS);
rwbase_restore_current_state();
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
return 0;
}
static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
{
struct rt_mutex_base *rtm = &rwb->rtmutex;
unsigned long flags;
if (!rwbase_rtmutex_trylock(rtm))
return 0;
atomic_sub(READER_BIAS, &rwb->readers);
raw_spin_lock_irqsave(&rtm->wait_lock, flags);
if (!atomic_read(&rwb->readers)) {
atomic_set(&rwb->readers, WRITER_BIAS);
raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
return 1;
}
__rwbase_write_unlock(rwb, 0, flags);
return 0;
}