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Mon, 3 Oct 2022 14:27:01 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id DiioCjXxOmMMaAAAMHmgww (envelope-from ); Mon, 03 Oct 2022 14:27:01 +0000 Date: Mon, 3 Oct 2022 16:27:00 +0200 From: Michal Hocko To: Alexander Fedorov Cc: Roman Gushchin , Johannes Weiner , Shakeel Butt , Vladimir Davydov , Muchun Song , Sebastian Andrzej Siewior , cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: Possible race in obj_stock_flush_required() vs drain_obj_stock() Message-ID: References: <1664546131660.1777662787.1655319815@gmail.com> <821923d8-17c3-f1c2-4d6a-5653c88db3e8@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <821923d8-17c3-f1c2-4d6a-5653c88db3e8@gmail.com> ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1664807223; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; 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dmarc=pass (policy=quarantine) header.from=suse.com; spf=pass (imf20.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.29 as permitted sender) smtp.mailfrom=mhocko@suse.com X-HE-Tag: 1664807222-512775 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 Mon 03-10-22 17:09:15, Alexander Fedorov wrote: > On 03.10.2022 16:32, Michal Hocko wrote: > > On Mon 03-10-22 15:47:10, Alexander Fedorov wrote: > >> @@ -3197,17 +3197,30 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock) > >> stock->nr_bytes = 0; > >> } > >> > >> - obj_cgroup_put(old); > >> + /* > >> + * Clear pointer before freeing memory so that > >> + * drain_all_stock() -> obj_stock_flush_required() > >> + * does not see a freed pointer. > >> + */ > >> stock->cached_objcg = NULL; > >> + obj_cgroup_put(old); > > > > Do we need barrier() or something else to ensure there is no reordering? > > I am not reallyu sure what kind of barriers are implied by the pcp ref > > counting. > > obj_cgroup_put() -> kfree_rcu() -> synchronize_rcu() should take care > of this: This is a very subtle guarantee. Also it would only apply if this is the last reference, right? Is there any reason to not use WRITE_ONCE(stock->cached_objcg, NULL); obj_cgroup_put(old); IIRC this should prevent any reordering. -- Michal Hocko SUSE Labs