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locking/qrwlock: Make use of _{acquire|release|relaxed}() atomics 

The qrwlock implementation is slightly heavy in its use of memory
barriers, mainly through the use of _cmpxchg() and _return() atomics, which
imply full barrier semantics.

This patch modifies the qrwlock code to use the more relaxed atomic
routines so that we can reduce the unnecessary barrier overhead on
weakly-ordered architectures.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hp.com
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1438880084-18856-7-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon 2015-08-06 17:54:42 +01:00 committed by Ingo Molnar
parent 2b2a85a4d3
commit 77e430e3e4
2 changed files with 18 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
include/asm-generic/qrwlock.h
View File

@ -68,7 +68,7 @@ static inline int queued_read_trylock(struct qrwlock *lock)
cnts = atomic_read(&lock->cnts);
if (likely(!(cnts & _QW_WMASK))) {
cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
cnts = (u32)atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
if (likely(!(cnts & _QW_WMASK)))
return 1;
atomic_sub(_QR_BIAS, &lock->cnts);
@ -89,8 +89,8 @@ static inline int queued_write_trylock(struct qrwlock *lock)
if (unlikely(cnts))
return 0;
return likely(atomic_cmpxchg(&lock->cnts,
cnts, cnts | _QW_LOCKED) == cnts);
return likely(atomic_cmpxchg_acquire(&lock->cnts,
cnts, cnts | _QW_LOCKED) == cnts);
}
/**
* queued_read_lock - acquire read lock of a queue rwlock
@ -100,7 +100,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
{
u32 cnts;
cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
if (likely(!(cnts & _QW_WMASK)))
return;
@ -115,7 +115,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
static inline void queued_write_lock(struct qrwlock *lock)
{
/* Optimize for the unfair lock case where the fair flag is 0. */
if (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)
if (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)
return;
queued_write_lock_slowpath(lock);
@ -130,8 +130,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
/*
* Atomically decrement the reader count
*/
smp_mb__before_atomic();
atomic_sub(_QR_BIAS, &lock->cnts);
(void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
}
/**

24
kernel/locking/qrwlock.c
View File

@ -55,7 +55,7 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
{
while ((cnts & _QW_WMASK) == _QW_LOCKED) {
cpu_relax_lowlatency();
cnts = smp_load_acquire((u32 *)&lock->cnts);
cnts = atomic_read_acquire(&lock->cnts);
}
}
@ -74,8 +74,9 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
* Readers in interrupt context will get the lock immediately
* if the writer is just waiting (not holding the lock yet).
* The rspin_until_writer_unlock() function returns immediately
* in this case. Otherwise, they will spin until the lock
* is available without waiting in the queue.
* in this case. Otherwise, they will spin (with ACQUIRE
* semantics) until the lock is available without waiting in
* the queue.
*/
rspin_until_writer_unlock(lock, cnts);
return;
@ -88,12 +89,11 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
arch_spin_lock(&lock->lock);
/*
* At the head of the wait queue now, increment the reader count
* and wait until the writer, if it has the lock, has gone away.
* At ths stage, it is not possible for a writer to remain in the
* waiting state (_QW_WAITING). So there won't be any deadlock.
* The ACQUIRE semantics of the following spinning code ensure
* that accesses can't leak upwards out of our subsequent critical
* section in the case that the lock is currently held for write.
*/
cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
rspin_until_writer_unlock(lock, cnts);
/*
@ -116,7 +116,7 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
/* Try to acquire the lock directly if no reader is present */
if (!atomic_read(&lock->cnts) &&
(atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
(atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
goto unlock;
/*
@ -127,7 +127,7 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
struct __qrwlock *l = (struct __qrwlock *)lock;
if (!READ_ONCE(l->wmode) &&
(cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
(cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
break;
cpu_relax_lowlatency();
@ -137,8 +137,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
for (;;) {
cnts = atomic_read(&lock->cnts);
if ((cnts == _QW_WAITING) &&
(atomic_cmpxchg(&lock->cnts, _QW_WAITING,
_QW_LOCKED) == _QW_WAITING))
(atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
_QW_LOCKED) == _QW_WAITING))
break;
cpu_relax_lowlatency();