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

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Message-ID: <5c99773f-448d-6ca7-e28e-c262a82ee58a@linux.intel.com>
Date:	Thu, 26 May 2016 13:08:58 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	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

Op 26-05-16 om 12:43 schreef Chris Wilson:
> 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)
That's true, but it cannot do the same when use_ww_ctx = true.
In this case the function will always be called with ww_ctx != NULL,
but the compiler can't see that, so it will keep the check even if it's always true.

~Maarten