linux-kernel - Re: [PATCH-tip v7 12/20] locking/rwsem: Clarify usage of owner's nonspinaable bit

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Date:   Fri, 3 May 2019 17:21:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH-tip v7 12/20] locking/rwsem: Clarify usage of owner's
 nonspinaable bit

On Sun, Apr 28, 2019 at 05:25:49PM -0400, Waiman Long wrote:
> Bit 1 of sem->owner (RWSEM_ANONYMOUSLY_OWNED) is used to designate an
> anonymous owner - readers or an anonymous writer. The setting of this
> anonymous bit is used as an indicator that optimistic spinning cannot
> be done on this rwsem.
> 
> With the upcoming reader optimistic spinning patches, a reader-owned
> rwsem can be spinned on for a limit period of time. We still need
> this bit to indicate a rwsem is nonspinnable, but not setting this
> bit loses its meaning that the owner is known. So rename the bit
> to RWSEM_NONSPINNABLE to clarify its meaning.
> 
> This patch also fixes a DEBUG_RWSEMS_WARN_ON() bug in __up_write().
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  include/linux/rwsem.h  |  2 +-
>  kernel/locking/rwsem.c | 43 +++++++++++++++++++++---------------------
>  2 files changed, 22 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 148983e21d47..bb76e82398b2 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -50,7 +50,7 @@ struct rw_semaphore {
>  };
>  
>  /*
> - * Setting bit 1 of the owner field but not bit 0 will indicate
> + * Setting all bits of the owner field except bit 0 will indicate
>   * that the rwsem is writer-owned with an unknown owner.
>   */
>  #define RWSEM_OWNER_UNKNOWN	((struct task_struct *)-2L)

As you know, I'm trying to kill that :-)