Messages in this thread Patch in this message | | | Date | Tue, 17 Nov 2020 09:30:16 +0100 | From | Peter Zijlstra <> | Subject | [PATCH] sched: Fix data-race in wakeup |
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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?
--- Subject: sched: Fix data-race in wakeup From: Peter Zijlstra <peterz@infradead.org> 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 <mgorman@techsingularity.net> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- 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;
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