linux-kernel - Re: [PATCH] locking/qrwlock: queued_write_lock

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Message-ID: <20210427075659.GA6675@willie-the-truck>
Date:   Tue, 27 Apr 2021 08:56:59 +0100
From:   Will Deacon <will@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Ali Saidi <alisaidi@...zon.com>,
        Steve Capper <steve.capper@....com>
Subject: Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup

On Mon, Apr 26, 2021 at 02:50:17PM -0400, Waiman Long wrote:
> Make the code more readable by replacing the atomic_cmpxchg_acquire()
> by an equivalent atomic_try_cmpxchg_acquire() and change atomic_add()
> to atomic_or().
> 
> For architectures that use qrwlock, I do not find one that has an
> atomic_add() defined but not an atomic_or().  I guess it should be fine
> by changing atomic_add() to atomic_or().
> 
> Note that the previous use of atomic_add() isn't wrong as only one
> writer that is the wait_lock owner can set the waiting flag and the
> flag will be cleared later on when acquiring the write lock.

Right, there's no functional change here.

> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/locking/qrwlock.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index b94f3831e963..ec36b73f4733 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -66,12 +66,12 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
>  	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))
> +	if (!(cnts = atomic_read(&lock->cnts)) &&
> +	    atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
>  		goto unlock;
>  
>  	/* Set the waiting flag to notify readers that a writer is pending */
> -	atomic_add(_QW_WAITING, &lock->cnts);
> +	atomic_or(_QW_WAITING, &lock->cnts);

Acked-by: Will Deacon <will@...nel.org>

Will