linux-kernel - Re: [PATCH] locking/rwsem: Remove unnessary check in rwsem_down_read

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Message-ID: <6da49b39-653a-1eda-2d21-1f1b50cb14f0@redhat.com>
Date:   Wed, 8 Nov 2023 09:04:48 -0500
From:   Waiman Long <longman@...hat.com>
To:     Haifeng Xu <haifeng.xu@...pee.com>
Cc:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        boqun.feng@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rwsem: Remove unnessary check in
 rwsem_down_read_slowpath()

On 11/8/23 05:56, Haifeng Xu wrote:
> When the owner of rw_semaphore is reader, the count can't be
> RWSEM_WRITER_LOCKED, so there is no need to check it.
>
> Signed-off-by: Haifeng Xu <haifeng.xu@...pee.com>
> ---
>   kernel/locking/rwsem.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 2340b6d90ec6..7a4d8a9ebd9c 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -1005,8 +1005,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
>   	 * waiter, don't attempt optimistic lock stealing if the lock is
>   	 * currently owned by readers.
>   	 */
> -	if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
> -	    (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
> +	if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) && (rcnt > 1))
>   		goto queue;
>   
>   	/*

Unlike RWSEM_WRITER_LOCKED bit in count, the RWSEM_READER_OWNED bit in 
owner is just a hint, not an authoritative state of the rwsem. So it is 
possible that both the RWSEM_READER_OWNED bit can be set in owner and 
RWSEM_WRITER_LOCKED bit set in count in a transition period right after 
RWSEM_WRITER_LOCKED bit is set. So the RWSEM_WRITER_LOCKED check can 
still provide some value. We should probably update the comment to 
reflect that.

Cheers,
Longman