linux-kernel - Re: [PATCH] mutex: Do not spin/queue before performing ww

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Message-ID: <20160526104329.GS15901@nuc-i3427.alporthouse.com>
Date:	Thu, 26 May 2016 11:43:29 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	intel-gfx@...ts.freedesktop.org,
	Christian König <christian.koenig@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mutex: Do not spin/queue before performing ww_mutex
 deadlock avoidance

On Thu, May 26, 2016 at 12:37:30PM +0200, Maarten Lankhorst wrote:
> The check should also not be for NULL, but for use_ww_ctx.
> This way the if check is optimized out for the ww_ctx path, where
> ww_ctx is always non-null.

The compiler can see use_ww_ctx == false => ww_ctx == NULL just as well
to do dead-code elimination, i.e. use_ww_ctx is superflouus and does not
reduce the code size. (gcc 4.7.2, 4.9.1, 5.3.1)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre