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Peter Zijlstra authored
Paul reported rcutorture occasionally hitting a NULL deref: sched_ttwu_pending() ttwu_do_wakeup() check_preempt_curr() := check_preempt_wakeup() find_matching_se() is_same_group() if (se->cfs_rq == pse->cfs_rq) <-- *BOOM* Debugging showed that this only appears to happen when we take the new code-path from commit: 2ebb1771 ("sched/core: Offload wakee task activation if it the wakee is descheduling") and only when @cpu == smp_processor_id(). Something which should not be possible, because p->on_cpu can only be true for remote tasks. Similarly, without the new code-path from commit: c6e7bd7a ("sched/core: Optimize ttwu() spinning on p->on_cpu") this would've unconditionally hit: smp_cond_load_acquire(&p->on_cpu, !VAL); and if: 'cpu == smp_processor_id() && p->on_cpu' is possible, this would result in an instant live-lock (with IRQs disabled), something that hasn't been reported. The NULL deref can be explained however if the task_cpu(p) load at the beginning of try_to_wake_up() returns an old value, and this old value happens to be smp_processor_id(). Further assume that the p->on_cpu load accurately returns 1, it really is still running, just not here. Then, when we enqueue the task locally, we can crash in exactly the observed manner because p->se.cfs_rq != rq->cfs_rq, because p's cfs_rq is from the wrong CPU, therefore we'll iterate into the non-existant parents and NULL deref. The closest semi-plausible scenario I've managed to contrive is somewhat elaborate (then again, actual reproduction takes many CPU hours of rcutorture, so it can't be anything obvious): X->cpu = 1 rq(1)->curr = X CPU0 CPU1 CPU2 // switch away from X LOCK rq(1)->lock smp_mb__after_spinlock dequeue_task(X) X->on_rq = 9 switch_to(Z) X->on_cpu = 0 UNLOCK rq(1)->lock // migrate X to cpu 0 LOCK rq(1)->lock dequeue_task(X) set_task_cpu(X, 0) X->cpu = 0 UNLOCK rq(1)->lock LOCK rq(0)->lock enqueue_task(X) X->on_rq = 1 UNLOCK rq(0)->lock // switch to X LOCK rq(0)->lock smp_mb__after_spinlock switch_to(X) X->on_cpu = 1 UNLOCK rq(0)->lock // X goes sleep X->state = TASK_UNINTERRUPTIBLE smp_mb(); // wake X ttwu() LOCK X->pi_lock smp_mb__after_spinlock if (p->state) cpu = X->cpu; // =? 1 smp_rmb() // X calls schedule() LOCK rq(0)->lock smp_mb__after_spinlock dequeue_task(X) X->on_rq = 0 if (p->on_rq) smp_rmb(); if (p->on_cpu && ttwu_queue_wakelist(..)) [*] smp_cond_load_acquire(&p->on_cpu, !VAL) cpu = select_task_rq(X, X->wake_cpu, ...) if (X->cpu != cpu) switch_to(Y) X->on_cpu = 0 UNLOCK rq(0)->lock However I'm having trouble convincing myself that's actually possible on x86_64 -- after all, every LOCK implies an smp_mb() there, so if ttwu observes ->state != RUNNING, it must also observe ->cpu != 1. (Most of the previous ttwu() races were found on very large PowerPC) Nevertheless, this fully explains the observed failure case. Fix it by ordering the task_cpu(p) load after the p->on_cpu load, which is easy since nothing actually uses @cpu before this. Fixes: c6e7bd7a ("sched/core: Optimize ttwu() spinning on p->on_cpu") Reported-by:
Paul E. McKenney <paulmck@kernel.org> Tested-by:
Paul E. McKenney <paulmck@kernel.org> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by:
Ingo Molnar <mingo@kernel.org> Link: https://lkml.kernel.org/r/20200622125649.GC576871@hirez.programming.kicks-ass.net
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