linux-kernel - Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem

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Message-ID: <20180515085111.GW12217@hirez.programming.kicks-ass.net>
Date:   Tue, 15 May 2018 10:51:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Oleg Nesterov <oleg@...hat.com>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as
 non-spinnable in percpu_rwsem_release()

On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote:
> The percpu_rwsem_release() is called when the ownership of the embedded
> rwsem is to be transferred to another task. The new owner, however, may
> take a while to get the ownership of the lock via percpu_rwsem_acquire().
> During that period, the rwsem is now marked as writer-owned with no
> optimistic spinning.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  include/linux/percpu-rwsem.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
> index b1f37a8..dd37102 100644
> --- a/include/linux/percpu-rwsem.h
> +++ b/include/linux/percpu-rwsem.h
> @@ -131,16 +131,16 @@ static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
>  					bool read, unsigned long ip)
>  {
>  	lock_release(&sem->rw_sem.dep_map, 1, ip);
> -#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>  	if (!read)
> -		sem->rw_sem.owner = NULL;
> -#endif
> +		rwsem_set_writer_owned_nospin(&sem->rw_sem);
>  }
>  
>  static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
>  					bool read, unsigned long ip)
>  {
>  	lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
> +	if (!read)
> +		rwsem_set_writer_owned(&sem->rw_sem, current);
>  }

So what's wrong with adding:

	if (!read)
		sem->rw_sem.owner = current;

?

Afaict the whole .owner=NULL thing in release already stops the spinners
dead, and the above 'fixes' the debug splat. And this avoids exposing
that horrible interface and keeps the mucking private to
rwsem/percpu_rwsem.