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Wed, 02 Jul 2025 21:45:47 -0700 (PDT) MIME-Version: 1.0 References: <20250702082749.141616-1-aha310510@gmail.com> <7b7d353f-f38b-3205-8fd4-1072dbf69cb6@gentwo.org> In-Reply-To: <7b7d353f-f38b-3205-8fd4-1072dbf69cb6@gentwo.org> From: Jeongjun Park Date: Thu, 3 Jul 2025 13:45:36 +0900 X-Gm-Features: Ac12FXxp251cGnSA30_Y2cOyMV6iAhuJkxr8Go3t_EWL2bdQaNtbK3UYggSR0jE Message-ID: Subject: Re: [PATCH] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock To: "Christoph Lameter (Ampere)" 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 Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: DF0361C0002 X-Stat-Signature: 9gu9fq9p8ao5mjdfhhkjs384prajqosy X-HE-Tag: 1751517948-853305 X-HE-Meta: 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 ud365Okv 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 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: Christoph Lameter (Ampere) wrote: > > 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? > > This patch is by no means an academic exercise. As written in the reported tag, This race has actually been reported in syzbot [1]. [1]: https://syzkaller.appspot.com/bug?extid=e5bd32b79413e86f389e pcpu_nr_populated is currently being write in pcpu_chunk_populated() and pcpu_chunk_depopulated(), and since this two functions perform pcpu_nr_populated write under the protection of pcpu_lock, there is no race for write/write. However, since pcpu_nr_pages(), which performs a read operation on pcpu_nr_populated, is not protected by pcpu_lock, races between read/write can easily occur. Therefore, I think it is appropriate to protect it through pcpu_lock according to the comment written in the definition of pcpu_nr_populated. -- Regards, Jeongjun Park