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Wed, 2 Jul 2025 08:55:13 -0700 (PDT) Date: Wed, 2 Jul 2025 08:55:13 -0700 (PDT) From: "Christoph Lameter (Ampere)" To: Jeongjun Park cc: dennis@kernel.org, tj@kernel.org, akpm@linux-foundation.org, vbabka@suse.cz, rientjes@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+e5bd32b79413e86f389e@syzkaller.appspotmail.com Subject: Re: [PATCH] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock In-Reply-To: <20250702082749.141616-1-aha310510@gmail.com> Message-ID: <7b7d353f-f38b-3205-8fd4-1072dbf69cb6@gentwo.org> References: <20250702082749.141616-1-aha310510@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Stat-Signature: wmp15b5gg6k148d7hx6w1mggnhozii3e X-Rspamd-Queue-Id: 353C0180009 X-Rspamd-Server: rspam11 X-Rspam-User: X-HE-Tag: 1751471714-271125 X-HE-Meta: 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 QJkD7RE9 cX4OA2BHZvExjdL0WVqieTZRNm05GYlQnIOCvX7F8mLqwag3wMFBhs9WfQiqhajGW2NCZ/EFYJ1N2MFUYAzj/nK9hpXiWi3hmAWRZPN8Yo6bXY4qDXIGb2DoMDicpF2pKO/Gc 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 Wed, 2 Jul 2025, Jeongjun Park wrote: > diff --git a/mm/percpu.c b/mm/percpu.c > index b35494c8ede2..0f98b857fb36 100644 > --- a/mm/percpu.c > +++ b/mm/percpu.c > @@ -3355,7 +3355,13 @@ void __init setup_per_cpu_areas(void) > */ > unsigned long pcpu_nr_pages(void) > { > - return pcpu_nr_populated * pcpu_nr_units; > + unsigned long flags, ret; > + > + spin_lock_irqsave(&pcpu_lock, flags); > + ret = pcpu_nr_populated * pcpu_nr_units; > + spin_unlock_irqrestore(&pcpu_lock, flags); Ummm.. What? You are protecting a single read with a spinlock? There needs to be some updating of data somewhere for this to make sense. Unless a different critical section protected by the lock sets the value intermittendly to something you are not allowed to see before a final store of a valid value. But that would be unusual. This is an academic exercise or did you really see a problem? What is racing?