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locking/qrwlock: Use atomic_cond_read_acquire() when spinning in qrwlock 

The qrwlock slowpaths involve spinning when either a prospective reader
is waiting for a concurrent writer to drain, or a prospective writer is
waiting for concurrent readers to drain. In both of these situations,
atomic_cond_read_acquire() can be used to avoid busy-waiting and make use
of any backoff functionality provided by the architecture.

This patch replaces the open-code loops and rspin_until_writer_unlock()
implementation with atomic_cond_read_acquire(). The write mode transition
zero to _QW_WAITING is left alone, since (a) this doesn't need acquire
semantics and (b) should be fast.

Tested-by: Waiman Long <longman@redhat.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Adam Wallis <awallis@codeaurora.org>
Tested-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jeremy.Linton@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1507810851-306-4-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon 2017-10-12 13:20:49 +01:00 committed by Ingo Molnar
parent 4df714be4d
commit b519b56e37
2 changed files with 14 additions and 40 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
include/asm-generic/qrwlock.h
View File

@ -49,7 +49,7 @@
/*
* External function declarations
*/
extern void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts);
extern void queued_read_lock_slowpath(struct qrwlock *lock);
extern void queued_write_lock_slowpath(struct qrwlock *lock);
/**
@ -100,7 +100,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
return;
/* The slowpath will decrement the reader count, if necessary. */
queued_read_lock_slowpath(lock, cnts);
queued_read_lock_slowpath(lock);
}
/**

50
kernel/locking/qrwlock.c
View File

@ -23,29 +23,11 @@
#include <linux/spinlock.h>
#include <asm/qrwlock.h>
/**
* rspin_until_writer_unlock - inc reader count & spin until writer is gone
* @lock : Pointer to queue rwlock structure
* @writer: Current queue rwlock writer status byte
*
* In interrupt context or at the head of the queue, the reader will just
* increment the reader count & wait until the writer releases the lock.
*/
static __always_inline void
rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
{
while ((cnts & _QW_WMASK) == _QW_LOCKED) {
cpu_relax();
cnts = atomic_read_acquire(&lock->cnts);
}
}
/**
* queued_read_lock_slowpath - acquire read lock of a queue rwlock
* @lock: Pointer to queue rwlock structure
* @cnts: Current qrwlock lock value
*/
void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
void queued_read_lock_slowpath(struct qrwlock *lock)
{
/*
* Readers come here when they cannot get the lock without waiting
@ -53,13 +35,12 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
if (unlikely(in_interrupt())) {
/*
* 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 (with ACQUIRE
* semantics) until the lock is available without waiting in
* the queue.
* if the writer is just waiting (not holding the lock yet),
* so spin with ACQUIRE semantics until the lock is available
* without waiting in the queue.
*/
rspin_until_writer_unlock(lock, cnts);
atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK)
!= _QW_LOCKED);
return;
}
atomic_sub(_QR_BIAS, &lock->cnts);
@ -68,14 +49,14 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
* Put the reader into the wait queue
*/
arch_spin_lock(&lock->wait_lock);
atomic_add(_QR_BIAS, &lock->cnts);
/*
* 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_fetch_add_acquire(_QR_BIAS, &lock->cnts);
rspin_until_writer_unlock(lock, cnts);
atomic_cond_read_acquire(&lock->cnts, (VAL & _QW_WMASK) != _QW_LOCKED);
/*
* Signal the next one in queue to become queue head
@ -90,8 +71,6 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
*/
void queued_write_lock_slowpath(struct qrwlock *lock)
{
u32 cnts;
/* Put the writer into the wait queue */
arch_spin_lock(&lock->wait_lock);
@ -113,15 +92,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
}
/* When no more readers, set the locked flag */
for (;;) {
cnts = atomic_read(&lock->cnts);
if ((cnts == _QW_WAITING) &&
(atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
_QW_LOCKED) == _QW_WAITING))
break;
cpu_relax();
}
do {
atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
_QW_LOCKED) != _QW_WAITING);
unlock:
arch_spin_unlock(&lock->wait_lock);
}