From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maarten Lankhorst Subject: Re: [PATCH] mutex: Do not spin/queue before performing ww_mutex deadlock avoidance Date: Thu, 26 May 2016 13:08:58 +0200 Message-ID: <5c99773f-448d-6ca7-e28e-c262a82ee58a@linux.intel.com> References: <1464251487-23778-1-git-send-email-chris@chris-wilson.co.uk> <20160526104329.GS15901@nuc-i3427.alporthouse.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160526104329.GS15901@nuc-i3427.alporthouse.com> Sender: linux-kernel-owner@vger.kernel.org To: Chris Wilson , Peter Zijlstra , Ingo Molnar , intel-gfx@lists.freedesktop.org, =?UTF-8?Q?Christian_K=c3=b6nig?= , linux-kernel@vger.kernel.org List-Id: intel-gfx@lists.freedesktop.org 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