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[71.184.117.43]) by smtp.gmail.com with ESMTPSA id z5sm1525247qta.7.2020.02.06.03.14.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Feb 2020 03:14:55 -0800 (PST) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: Qian Cai Mime-Version: 1.0 (1.0) Subject: Re: [PATCH -next] mm: mark a intentional data race in page_zonenum() Date: Thu, 6 Feb 2020 06:14:55 -0500 Message-Id: <3E1FB0B8-F499-45F7-9B03-7B80395FFF19@lca.pw> References: <20200206090436.GF14001@quack2.suse.cz> Cc: John Hubbard , akpm@linux-foundation.org, ira.weiny@intel.com, dan.j.williams@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org In-Reply-To: <20200206090436.GF14001@quack2.suse.cz> To: Jan Kara X-Mailer: iPhone Mail (17C54) 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: > On Feb 6, 2020, at 4:04 AM, Jan Kara wrote: >=20 > So in this particular case, store tearing is non-issue because we use > atomic operation to store the value in page_cpupid_xchg_last(). I think it= > would make some sense to use READ_ONCE(page->flags) here to prevent > compiler from loading page->flags several times - I have hard time finding= > a reason why a compiler would want to do that but conceptually that > protection makes sense, it is for free performance wise, and will still > allow KCSAN to find a race in case we ever grow a place that modifies > page's zone non-atomically (which might be a real problem). And it should > also silence the KCSAN warning AFAIU. Ah, read up to 3 bits might be an issue then. I=E2=80=99ll post an alternati= ve version which uses READ_ONCE() just for the old page ( because the new pa= ge has not been published yet) in wp_page_copy() then.=