linux-kernel - Re: [PATCH] mutex: Remove ww_ctx unlikely() from __mutex_lock

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Date:   Thu, 19 Jan 2017 17:42:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mutex: Remove ww_ctx unlikely() from
 __mutex_lock_common()

On Thu, Jan 19, 2017 at 08:54:41AM -0500, Steven Rostedt (VMware) wrote:
> I added a comment stating that the branch should be unlikely, but due
> to the intel drm logic, it currently isn't. Then if drm changes in the
> future, we could then try it again.

I really don't see the point here. The unlikely() also conveys this is
not a fast path branch and that is still true, regardsless of what
runtime does.

Also, the patch wouldn't apply even if I were so inclined.

> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index a70b90d..577bb74 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -513,7 +513,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  
>  	if (use_ww_ctx) {
>  		struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
> -		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
> +		/*
> +		 * This really should be an unlikely() but currently
> +		 * the intel drm makes this a very likely case.
> +		 */
> +		if (ww_ctx == READ_ONCE(ww->ctx))
>  			return -EALREADY;
>  	}
>