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Tue, 17 Nov 2020 09:30:16 +0100 (CET) Date: Tue, 17 Nov 2020 09:30:16 +0100 From: Peter Zijlstra To: Mel Gorman Subject: [PATCH] sched: Fix data-race in wakeup Message-ID: <20201117083016.GK3121392@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> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201116193149.GW3371@techsingularity.net> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Davidlohr Bueso , Will Deacon , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Nov 16, 2020 at 07:31:49PM +0000, Mel Gorman wrote: > And this works. Yay! > sched_psi_wake_requeue can probably stay with the other three fields > given they are under the rq lock but sched_remote_wakeup needs to move > out. I _think_ we can move the bit into the unserialized section below. It's a bit cheecky, but it should work I think because the only time we actually use this bit, we're guaranteed the task isn't actually running, so current doesn't exist. I suppose the question is wether this is worth saving 31 bits over... How's this? Acked-by: Will Deacon Reviewed-by: Mel Gorman Reviewed-by: Valentin Schneider --- 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); 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. + */ + unsigned sched_remote_wakeup:1; + /* Bit to tell LSMs we're in execve(): */ unsigned in_execve:1; unsigned in_iowait:1;