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Tue, 17 Nov 2020 09:29:39 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 26D2D3019CE; Tue, 17 Nov 2020 10:29:36 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 12D382012638E; Tue, 17 Nov 2020 10:29:36 +0100 (CET) Date: Tue, 17 Nov 2020 10:29:36 +0100 From: Peter Zijlstra To: Will Deacon Cc: Mel Gorman , Davidlohr Bueso , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: Fix data-race in wakeup Message-ID: <20201117092936.GA3121406@hirez.programming.kicks-ass.net> References: <20201116091054.GL3371@techsingularity.net> <20201116131102.GA29992@willie-the-truck> <20201116133721.GQ3371@techsingularity.net> <20201116142005.GE3121392@hirez.programming.kicks-ass.net> <20201116193149.GW3371@techsingularity.net> <20201117083016.GK3121392@hirez.programming.kicks-ass.net> <20201117091545.GA31837@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201117091545.GA31837@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 17, 2020 at 09:15:46AM +0000, Will Deacon wrote: > On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote: > > Subject: sched: Fix data-race in wakeup > > From: Peter Zijlstra > > Date: Tue Nov 17 09:08:41 CET 2020 > > > > Mel reported that on some ARM64 platforms loadavg goes bananas and > > tracked it down to the following data race: > > > > CPU0 CPU1 > > > > schedule() > > prev->sched_contributes_to_load = X; > > deactivate_task(prev); > > > > try_to_wake_up() > > if (p->on_rq &&) // false > > if (smp_load_acquire(&p->on_cpu) && // true > > ttwu_queue_wakelist()) > > p->sched_remote_wakeup = Y; > > > > smp_store_release(prev->on_cpu, 0); > > (nit: I suggested this race over at [1] ;) Ah, I'll ammend and get you a Debugged-by line or something ;-) > > where both p->sched_contributes_to_load and p->sched_remote_wakeup are > > in the same word, and thus the stores X and Y race (and can clobber > > one another's data). > > > > Whereas prior to commit c6e7bd7afaeb ("sched/core: Optimize ttwu() > > spinning on p->on_cpu") the p->on_cpu handoff serialized access to > > p->sched_remote_wakeup (just as it still does with > > p->sched_contributes_to_load) that commit broke that by calling > > ttwu_queue_wakelist() with p->on_cpu != 0. > > > > However, due to > > > > p->XXX ttwu() > > schedule() if (p->on_rq && ...) // false > > smp_mb__after_spinlock() if (smp_load_acquire(&p->on_cpu) && > > deactivate_task() ttwu_queue_wakelist()) > > p->on_rq = 0; p->sched_remote_wakeup = X; > > > > We can be sure any 'current' store is complete and 'current' is > > guaranteed asleep. Therefore we can move p->sched_remote_wakeup into > > the current flags word. > > > > Note: while the observed failure was loadavg accounting gone wrong due > > to ttwu() cobbering p->sched_contributes_to_load, the reverse problem > > is also possible where schedule() clobbers p->sched_remote_wakeup, > > this could result in enqueue_entity() wrecking ->vruntime and causing > > scheduling artifacts. > > > > Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu") > > Reported-by: Mel Gorman > > Signed-off-by: Peter Zijlstra (Intel) > > --- > > include/linux/sched.h | 13 ++++++++++++- > > 1 file changed, 12 insertions(+), 1 deletion(-) > > > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -775,7 +775,6 @@ struct task_struct { > > unsigned sched_reset_on_fork:1; > > unsigned sched_contributes_to_load:1; > > unsigned sched_migrated:1; > > - unsigned sched_remote_wakeup:1; > > #ifdef CONFIG_PSI > > unsigned sched_psi_wake_requeue:1; > > #endif > > @@ -785,6 +784,18 @@ struct task_struct { > > > > /* Unserialized, strictly 'current' */ > > > > + /* > > + * p->in_iowait = 1; ttwu() > > + * schedule() if (p->on_rq && ..) // false > > + * smp_mb__after_spinlock(); if (smp_load_acquire(&p->on_cpu) && //true > > + * deactivate_task() ttwu_queue_wakelist()) > > + * p->on_rq = 0; p->sched_remote_wakeup = X; > > + * > > + * Guarantees all stores of 'current' are visible before > > + * ->sched_remote_wakeup gets used. > > I'm still not sure this is particularly clear -- don't we want to highlight > that the store of p->on_rq is unordered wrt the update to > p->sched_contributes_to_load() in deactivate_task()? I can explicitly call that out I suppose. > I dislike bitfields with a passion, but the fix looks good: I don't particularly hate them, they're just a flag field with names on (in this case). > Acked-by: Will Deacon Thanks! > Now the million dollar question is why KCSAN hasn't run into this. Hrmph. kernel/sched/Makefile:KCSAN_SANITIZE := n might have something to do with that, I suppose.