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Wed, 13 Aug 2025 23:44:40 -0700 (PDT) Message-ID: Date: Thu, 14 Aug 2025 07:44:40 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] drm/i915/active: Use try_cmpxchg64() in __active_lookup() To: Uros Bizjak , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Cc: Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , David Airlie , Simona Vetter References: <20250725072727.68486-1-ubizjak@gmail.com> Content-Language: en-GB From: Tvrtko Ursulin In-Reply-To: <20250725072727.68486-1-ubizjak@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi, On 25/07/2025 08:26, Uros Bizjak wrote: > Replace this pattern in __active_lookup(): > > cmpxchg64(*ptr, old, new) == old > > ... with the simpler and faster: > > try_cmpxchg64(*ptr, &old, new) > > The x86 CMPXCHG instruction returns success in the ZF flag, > so this change saves a compare after the CMPXCHG. > > The patch also improves the explanation of what the code really > does. cmpxchg64() will *succeed* for the winner of the race and > try_cmpxchg64() nicely documents this fact. > > No functional change intended. > > Signed-off-by: Uros Bizjak > Cc: Jani Nikula > Cc: Joonas Lahtinen > Cc: Rodrigo Vivi > Cc: Tvrtko Ursulin > Cc: David Airlie > Cc: Simona Vetter > --- > drivers/gpu/drm/i915/i915_active.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c > index 0dbc4e289300..6b0c1162505a 100644 > --- a/drivers/gpu/drm/i915/i915_active.c > +++ b/drivers/gpu/drm/i915/i915_active.c > @@ -257,10 +257,9 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx) > * claimed the cache and we know that is does not match our > * idx. If, and only if, the timeline is currently zero is it > * worth competing to claim it atomically for ourselves (for > - * only the winner of that race will cmpxchg return the old > - * value of 0). > + * only the winner of that race will cmpxchg succeed). > */ > - if (!cached && !cmpxchg64(&it->timeline, 0, idx)) > + if (!cached && try_cmpxchg64(&it->timeline, &cached, idx)) > return it; > } > Patch looks fine, thank you! I've sent it for a CI pass (see https://patchwork.freedesktop.org/series/152185/) before merging. Regards, Tvrtko