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Fri, 14 Mar 2025 21:08:56 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id wTxLOeia1GcRJwAAD6G6ig (envelope-from ); Fri, 14 Mar 2025 21:08:56 +0000 Message-ID: Date: Fri, 14 Mar 2025 22:08:56 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH bpf-next v9 1/6] locking/local_lock: Introduce localtry_lock_t To: Alexei Starovoitov Cc: Mateusz Guzik , Sebastian Andrzej Siewior , bpf , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Hou Tao , Johannes Weiner , Shakeel Butt , Michal Hocko , Matthew Wilcox , Thomas Gleixner , Jann Horn , Tejun Heo , linux-mm , Kernel Team References: <20250222024427.30294-1-alexei.starovoitov@gmail.com> <20250222024427.30294-2-alexei.starovoitov@gmail.com> <20250311162059.BunTzxde@linutronix.de> <496ff0d2-97ac-41f5-a776-455025fb72db@suse.cz> Content-Language: en-US From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 21268C0008 X-Stat-Signature: p86oybr5e7s743aazm75b35ha5bct6p7 X-HE-Tag: 1741986538-23932 X-HE-Meta: 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 bjD3ktHW tBzu46Pxw3gfsfzkuF/T0idKXmoATmQhKhPbGVSlAxAg7seN8Aaiy1YMzR3hGAgWBzR1eb0KEPb1CJwKZNxJERbjwjW+SUFf3QqlJL+gvSC5/xWAx5voSk8z4K3c22ypsgAJipjzWM70xDSz5gtNQ6DegFx9Wro9SxvtbLwiLAFTRrtjPudR/g368GUbTTuqHWy3ZBbOZZmJLaOnd18ceB18ObFO0yBe3PngvF5gN9jIeLMjoIR6SMJcap9z8RAJzpezg4Ytqb0GFpA89mcrBXlABjsg8ROW/Uj0VhXdxukk6pKFy8iEjm621UyjJMW4iRKdYggtwJh1PySCf99pA3cavmaKkYwoHViGsM8kNsRqwwcTTHugb/dW7toso36EeozHi0DvcFGvF7t8lqSKAZlJDDIuqtWfXk1CVPccaaUeSPgY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/14/25 22:05, Alexei Starovoitov wrote: > On Wed, Mar 12, 2025 at 1:29 AM Vlastimil Babka wrote: > > That's correct. > >> An if we e.g. have a pointer to memcg_stock_pcp through which we access the >> stock_lock an the other (protected) fields and that pointer doesn't change >> between that, I imagine gcc can reliably determine these can't alias? > > Though my last gcc commit was very long ago here is a simple example > where compiler can reorder/combine stores: > struct s { > short a, b; > } *p; > p->a = 1; > p->b = 2; > The compiler can keep them as-is, combine or reorder even with > -fno-strict-aliasing, because it can determine that a and b don't alias. > > But after re-reading gcc doc on volatiles again it's clear that > extra barriers are not necessary. > The main part: > "The minimum requirement is that at a sequence point all previous > accesses to volatile objects have stabilized" > > So anything after WRITE_ONCE(lt->acquired, 1); will not be hoisted up > and that's what we care about here. OK, is there similar guarantee for the unlock side? No write will be moved after WRITE_ONCE(lt->acquired, 0); there?