From: Waiman Long <llong@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org, Borislav Petkov <bp@suse.de>,
Ali Saidi <alisaidi@amazon.com>,
Steve Capper <steve.capper@arm.com>
Subject: Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup
Date: Mon, 26 Apr 2021 10:54:41 -0400 [thread overview]
Message-ID: <d8b2bc8a-3788-6cba-ffa9-2985d7a278d4@redhat.com> (raw)
In-Reply-To: <YIZ0+hNh0SMQoOkh@hirez.programming.kicks-ass.net>
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
next prev parent reply other threads:[~2021-04-26 14:54 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-25 20:06 [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup Waiman Long
2021-04-26 8:08 ` Peter Zijlstra
2021-04-26 14:54 ` Waiman Long [this message]
2021-04-26 18:50 Waiman Long
2021-04-26 18:51 ` Waiman Long
2021-04-27 7:56 ` Will Deacon
2021-05-04 9:19 ` Peter Zijlstra
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