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Date: Tue, 16 Dec 2014 14:00:40 +0800 From: Baoquan He <bhe@...hat.com> To: linux-kernel@...r.kernel.org Cc: mingo@...hat.com, peterz@...radead.org, Waiman.Long@...com, Baoquan He <bhe@...hat.com> Subject: [PATCH] locking/rwlocks: clean up of qrwlock In queue_read_lock_slowpath, when writer count becomes 0, we need increment the read count and get the lock. Then need call rspin_until_writer_unlock to check again if an incoming writer steals the lock in the gap. But in rspin_until_writer_unlock it only checks the writer count, namely low 8 bit of lock->cnts, no need to subtract the reader count unit specifically. So remove that subtraction to make it clearer, rspin_until_writer_unlock just takes the actual lock->cnts as the 2nd argument. And also change the code comment in queue_write_lock_slowpath to make it more exact and explicit. Signed-off-by: Baoquan He <bhe@...hat.com> --- kernel/locking/qrwlock.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c index f956ede..ae66c10 100644 --- a/kernel/locking/qrwlock.c +++ b/kernel/locking/qrwlock.c @@ -76,7 +76,7 @@ void queue_read_lock_slowpath(struct qrwlock *lock) while (atomic_read(&lock->cnts) & _QW_WMASK) cpu_relax_lowlatency(); - cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS; + cnts = atomic_add_return(_QR_BIAS, &lock->cnts); rspin_until_writer_unlock(lock, cnts); /* @@ -97,14 +97,14 @@ void queue_write_lock_slowpath(struct qrwlock *lock) /* Put the writer into the wait queue */ arch_spin_lock(&lock->lock); - /* Try to acquire the lock directly if no reader is present */ + /* Try to acquire the lock directly if no reader and writer is present */ if (!atomic_read(&lock->cnts) && (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)) goto unlock; /* - * Set the waiting flag to notify readers that a writer is pending, - * or wait for a previous writer to go away. + * Wait for a previous writer to go away, then set the waiting flag to + * notify readers that a writer is pending. */ for (;;) { cnts = atomic_read(&lock->cnts); -- 1.8.5.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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