From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maarten Lankhorst Subject: Re: [PATCH 4/4] drm/ttm: Optimize reservation slightly Date: Mon, 05 Nov 2012 15:01:05 +0100 Message-ID: <5097C6A1.9000405@canonical.com> References: <1352122278-12896-1-git-send-email-thellstrom@vmware.com> <1352122278-12896-5-git-send-email-thellstrom@vmware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1352122278-12896-5-git-send-email-thellstrom@vmware.com> Sender: linux-kernel-owner@vger.kernel.org To: Thomas Hellstrom Cc: airlied@gmail.com, airlied@redhat.com, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org List-Id: dri-devel@lists.freedesktop.org Hey, Op 05-11-12 14:31, Thomas Hellstrom schreef: > Reservation locking currently always takes place under the LRU spinlock. > Hence, strictly there is no need for an atomic_cmpxchg call; we can use > atomic_read followed by atomic_write since nobody else will ever reserve > without the lru spinlock held. > At least on Intel this should remove a locked bus cycle on successful > reserve. > > Signed-off-by: Thomas Hellstrom > Is that really a good thing to submit when I am busy killing lru lock around reserve? :-) - while (unlikely(atomic_cmpxchg(&bo->reserved, 0, 1) != 0)) { + while (unlikely(atomic_xchg(&bo->reserved, 1) != 0)) { Works without lru lock too! In fact mutexes are done in a similar way[1], except with some more magic, and unlocked state is 1, not 0. However I do think that to get that right (saves a irq disable in unlock path, and less wakeups in contended case), I should really just post the mutex extension patches for reservations and ride the flames. It's getting too close to real mutexes so I really want it to be a mutex in that case. So lets convert it.. Soon! :-) ~Maarten [1] See linux/include/asm-generic/mutex-xchg.h and linux/include/asm-generic/mutex-dec.h for how archs generally implement mutex fastpaths.