Messages in this thread | | | Date | Mon, 26 Apr 2021 10:08:26 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup |
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On Sun, Apr 25, 2021 at 04:06:37PM -0400, Waiman Long wrote:
> void queued_write_lock_slowpath(struct qrwlock *lock) > { > - int cnts; > + int cnts = 0; > > /* Put the writer into the wait queue */ > arch_spin_lock(&lock->wait_lock); > > /* Try to acquire the lock directly if no reader is present */ > if (!atomic_read(&lock->cnts) && > - (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)) > + atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)) > goto unlock;
Would not something like:
if (!(cnts = atomic_read(&lock->cnts)) && atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED) goto unlock;
Be clearer?
> > - /* Set the waiting flag to notify readers that a writer is pending */ > - atomic_add(_QW_WAITING, &lock->cnts); > + /* > + * Set the waiting flag to notify readers that a writer is pending > + * > + * As only one writer who is the wait_lock owner can set the waiting > + * flag which will be cleared later on when acquiring the write lock, > + * we can easily replace atomic_or() by an atomic_add() if there is > + * an architecture where an atomic_add() performs better than an > + * atomic_or().
That might be a little overboard on the comment, but sure :-) I don't think there's any arch that doesn't have atomic_or(), like I wrote elsewhere, the one that's often an issue is atomic_fetch_or().
> + */ > + atomic_or(_QW_WAITING, &lock->cnts);
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