Re: [PATCH] mutex: Do not spin/queue before performing ww_mutex deadlock avoidance

From: Maarten Lankhorst
Date: Thu May 26 2016 - 07:09:09 EST

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.