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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49e7ef2e117sm54270165e9.1.2026.09.15.03.20.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Sep 2026 03:20:59 -0700 (PDT) Date: Tue, 15 Sep 2026 11:20:58 +0100 From: David Laight To: Peter Zijlstra Cc: Waiman Long , Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, Linus Torvalds , Yafang Shao , Steven Rostedt Subject: Re: [PATCH v4 next 3/9] locking/osq_lock: Set prev_cpu=0 instead of locked=1 Message-ID: <20260915112058.309b5317@pumpkin> In-Reply-To: <20260915085013.GA4121339@noisy.programming.kicks-ass.net> References: <20260907084133.3696-1-david.laight.linux@gmail.com> <20260907084133.3696-4-david.laight.linux@gmail.com> <20260915085013.GA4121339@noisy.programming.kicks-ass.net> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 15 Sep 2026 10:50:13 +0200 Peter Zijlstra wrote: > On Mon, Sep 07, 2026 at 09:41:27AM +0100, David Laight wrote: > > There is no need for separate prev_cpu and locked members of > > struct optimistic_spin_node. > > Using a single field simplifies the code slightly. > > It also removes any possibility of the two values being out of sync. > > > > When cancelling a lock request explicitly set prev_cpu to zero. > > Nothing actually looks at the field, but it means that it will be zero > > after a subsequent 'fast path' osq_lock() call making things consistent. > > The cache line is likely to be dirty (or be dirtied) so there shouldn't > > be a performance hit. > > > > Signed-off-by: David Laight > > --- > > > --- a/kernel/locking/osq_lock.c > +++ b/kernel/locking/osq_lock.c > @@ -170,10 +170,11 @@ bool osq_lock(struct optimistic_spin_que > * is per-cpu data the memory can always be read. > */ > > - for (;; prev = READ_ONCE(node->prev)) { > - if (!prev) > - /* Lock acquired */ > + for (;;) { > + if (!prev) { > + smp_acquire__after_ctrl_dep(); Do you think the equivalent might fix the existing code? I think it is only going to matter if there isn't another read barrier before the osq_unlock() - which is unlikely except for the test program. > return true; > + } > > prev_ptr = decode_cpu(prev); > > @@ -185,8 +186,8 @@ bool osq_lock(struct optimistic_spin_que > * 'prev' must have unlinked (or be in the process of unlinking) > * itself from the list. > */ > - > cpu_relax(); > + prev = READ_ONCE(node->prev); A matter of taste, I prefer it at the top. David > } > > /*