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From: Roman Gushchin To: Alexander Fedorov Cc: Johannes Weiner , Michal Hocko , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1664727417; 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; bh=0nWc8MunwJqD9KPrKGnLf0JLwwF80l+3rGQtQR1k1PY=; b=M/jgRAvH8xHFxZu43pm/QEBNT1LcN46ZwaiHeRs6jOC8d9EsHIUYssju9OKCaqxv1KUsLC UcVvh7rT26vCaw+sScx1z4WTF11rqnTyvJkE8Pf3BlK/rAYoafyGDm4GGp9b/glqUEJW6k O7xzOfsuita2Ew7FWEOj9OXpZ7FhFOQ= ARC-Authentication-Results: i=1; imf23.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="Lyp+X/s8"; spf=pass (imf23.hostedemail.com: domain of roman.gushchin@linux.dev designates 91.121.223.63 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1664727417; a=rsa-sha256; cv=none; b=ILhwRRxtyvrxXUlMVjV1wJ2+KNBecVqtrikv0YX1LIU8gXFzP2dPcVdVxnn5Ot2v19hryV IxgOe11lbz2r+cF7ImDv/JRmuAzmGHdpUmxhfEnMtsOOiwH4ce5FzvAHcAPMOzEFcSa7Mb VqZYMQDO/0vCDGsj6aLHo0yKdDZni/Q= Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="Lyp+X/s8"; spf=pass (imf23.hostedemail.com: domain of roman.gushchin@linux.dev designates 91.121.223.63 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev; dmarc=pass (policy=none) header.from=linux.dev X-Rspam-User: X-Stat-Signature: 93aq1j3mzyo3h3yukdw1f7u1hup47nqq X-Rspamd-Queue-Id: 4AF59140014 X-Rspamd-Server: rspam08 X-HE-Tag: 1664727417-771779 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 Sat, Oct 01, 2022 at 03:38:43PM +0300, Alexander Fedorov wrote: > On 30.09.2022 21:26, Roman Gushchin wrote: > > On Fri, Sep 30, 2022 at 02:06:48PM +0000, Alexander Fedorov wrote: > >> 1) First CPU: > >> css_killed_work_fn() -> mem_cgroup_css_offline() -> > >> drain_all_stock() -> obj_stock_flush_required() > >> if (stock->cached_objcg) { > >> > >> This check sees a non-NULL pointer for *another* CPU's `memcg_stock` > >> instance. > >> > >> 2) Second CPU: > >> css_free_rwork_fn() -> __mem_cgroup_free() -> free_percpu() -> > >> obj_cgroup_uncharge() -> drain_obj_stock() > >> It frees `cached_objcg` pointer in its own `memcg_stock` instance: > >> struct obj_cgroup *old = stock->cached_objcg; > >> < ... > > >> obj_cgroup_put(old); > >> stock->cached_objcg = NULL; > >> > >> 3) First CPU continues after the 'if' check and re-reads the pointer > >> again, now it is NULL and dereferencing it leads to kernel panic: > >> static bool obj_stock_flush_required(struct memcg_stock_pcp *stock, > >> struct mem_cgroup *root_memcg) > >> { > >> < ... > > >> if (stock->cached_objcg) { > >> memcg = obj_cgroup_memcg(stock->cached_objcg); > > > > Great catch! > > > > I'm not sure about switching to rcu primitives though. In all other cases > > stock->cached_objcg is accessed only from a local cpu, so using rcu_* > > function is an overkill. > > > > How's something about this? (completely untested) > > Tested READ_ONCE() patch and it works. Thank you! > But are rcu primitives an overkill? > For me they are documenting how actually complex is synchronization here. I agree, however rcu primitives will add unnecessary barriers on hot paths. In this particular case most accesses to stock->cached_objcg are done from a local cpu, so no rcu primitives are needed. So in my opinion using a READ_ONCE() is preferred. Thanks!