Re: [PATCH] mutex: Remove ww_ctx unlikely() from __mutex_lock_common()

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] mutex: Remove ww_ctx unlikely() from __mutex_lock_common()
Date	2017-01-19 22:00 +0100
Message-ID	<t1oe7-40V-53@gated-at.bofh.it> (permalink)
References	<t15Et-O4-3@gated-at.bofh.it> <t1gJA-7Hu-3@gated-at.bofh.it> <t1lzz-2wM-25@gated-at.bofh.it>
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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@goodmis.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;
>  	}
>

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__mutex_lock_common() unlikely very likely Steven Rostedt <rostedt@goodmis.org> - 2017-01-18 22:10 +0100
  Re: __mutex_lock_common() unlikely very likely Chris Wilson <chris@chris-wilson.co.uk> - 2017-01-19 10:00 +0100
    Re: __mutex_lock_common() unlikely very likely Steven Rostedt <rostedt@goodmis.org> - 2017-01-19 14:40 +0100
    [PATCH] mutex: Remove ww_ctx unlikely() from __mutex_lock_common() "Steven Rostedt (VMware)" <rostedt@goodmis.org> - 2017-01-19 15:10 +0100
      Re: [PATCH] mutex: Remove ww_ctx unlikely() from  __mutex_lock_common() Steven Rostedt <rostedt@goodmis.org> - 2017-01-19 18:00 +0100
      Re: [PATCH] mutex: Remove ww_ctx unlikely() from  __mutex_lock_common() Peter Zijlstra <peterz@infradead.org> - 2017-01-19 22:00 +0100

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