Messages in this thread | | | From | Waiman Long <> | Subject | Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup | Date | Mon, 26 Apr 2021 10:54:41 -0400 |
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On 4/26/21 4:08 AM, Peter Zijlstra wrote: > 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? That works for me too. It is equivalent anyway. > >> >> - /* 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(). > I was not sure as I didn't look at other archs that hadn't used qrwlock yet. Given what you said, I will remove the comment.
Cheers, Longman
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