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[194.69.102.21]) by smtp.gmail.com with ESMTPSA id h7sm19467307wrs.15.2019.10.08.07.20.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 08 Oct 2019 07:20:40 -0700 (PDT) Date: Tue, 8 Oct 2019 16:20:35 +0200 From: Andrea Parri To: Dmitry Vyukov Cc: Christian Brauner , bsingharora@gmail.com, Marco Elver , LKML , syzbot , syzkaller-bugs , stable Subject: Re: [PATCH v2] taskstats: fix data-race Message-ID: <20191008142035.GA13564@andrea.guest.corp.microsoft.com> References: <20191007104039.GA16085@andrea.guest.corp.microsoft.com> <20191007110117.1096-1-christian.brauner@ubuntu.com> <20191007131804.GA19242@andrea.guest.corp.microsoft.com> <20191007141432.GA22083@andrea.guest.corp.microsoft.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 07, 2019 at 04:18:26PM +0200, Dmitry Vyukov wrote: > On Mon, Oct 7, 2019 at 4:14 PM Andrea Parri wrote: > > > > > > > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > > > > > { > > > > > struct signal_struct *sig = tsk->signal; > > > > > - struct taskstats *stats; > > > > > + struct taskstats *stats_new, *stats; > > > > > > > > > > - if (sig->stats || thread_group_empty(tsk)) > > > > > - goto ret; > > > > > + /* Pairs with smp_store_release() below. */ > > > > > + stats = READ_ONCE(sig->stats); > > > > > > > > This pairing suggests that the READ_ONCE() is heading an address > > > > dependency, but I fail to identify it: what is the target memory > > > > access of such a (putative) dependency? > > > > > > I would assume callers of this function access *stats. So the > > > dependency is between loading stats and accessing *stats. > > > > AFAICT, the only caller of the function in 5.4-rc2 is taskstats_exit(), > > which 'casts' the return value to a boolean (so I really don't see how > > any address dependency could be carried over/relied upon here). > > This does not make sense. > > But later taskstats_exit does: > > memcpy(stats, tsk->signal->stats, sizeof(*stats)); > > Perhaps it's supposed to use stats returned by taskstats_tgid_alloc? Seems reasonable to me. If so, replacing the READ_ONCE() in question with an smp_load_acquire() might be the solution. Thoughts? Andrea