Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH v3 10/19] sched: Fix migrate_disable() vs set_cpus_allowed_ptr() | Date | Fri, 16 Oct 2020 13:48:17 +0100 |
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On 15/10/20 12:05, Peter Zijlstra wrote: > @@ -1862,15 +1875,27 @@ static int migration_cpu_stop(void *data > * we're holding p->pi_lock. > */ > if (task_rq(p) == rq) { > + if (is_migration_disabled(p)) > + goto out; > + > if (task_on_rq_queued(p)) > rq = __migrate_task(rq, &rf, p, arg->dest_cpu); > else > p->wake_cpu = arg->dest_cpu; > + > + if (arg->done) { > + p->migration_pending = NULL; > + complete = true;
Ok so nasty ahead:
P0@CPU0 P1 P2 stopper
migrate_disable(); sca(P0, {CPU1}); <installs pending> migrate_enable(); <kicks stopper> sca(P0, {CPU0}); <locks> <local, has pending: goto do_complete> <unlocks> complete_all(); refcount_dec(); refcount_dec(); <done> <done>
<locks> <fiddles with pending->arg->done>
First, P2 can clear p->migration_pending before the stopper gets to run.
Second, the complete_all() is done without pi / rq locks held, but P2 might get to it before the stopper does. This may cause &pending to be popped off the stack before the stopper gets to it, so mayhaps we would need the below hunk.
The move_queued_task() from the stopper is "safe" in that we won't kick a task outside of its allowed mask, although we may move it around for no reason - I tried to prevent that.
--- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a5b6eac07adb..1ebf653c2c2f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1859,6 +1859,13 @@ static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf, return rq; } +struct set_affinity_pending { + refcount_t refs; + struct completion done; + struct cpu_stop_work stop_work; + struct migration_arg arg; +}; + /* * migration_cpu_stop - this will be executed by a highprio stopper thread * and performs thread migration by bumping thread off CPU then @@ -1866,6 +1873,7 @@ static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf, */ static int migration_cpu_stop(void *data) { + struct set_affinity_pending *pending; struct migration_arg *arg = data; struct task_struct *p = arg->task; struct rq *rq = this_rq(); @@ -1886,13 +1894,22 @@ static int migration_cpu_stop(void *data) raw_spin_lock(&p->pi_lock); rq_lock(rq, &rf); + + if (arg->done) + pending = container_of(arg->done, struct set_affinity_pending, done); /* * If task_rq(p) != rq, it cannot be migrated here, because we're * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because * we're holding p->pi_lock. */ if (task_rq(p) == rq) { - if (is_migration_disabled(p)) + /* + * An affinity update may have raced with us. + * p->migration_pending could now be NULL, or could be pointing + * elsewhere entirely. + */ + if (is_migration_disabled(p) || + (arg->done && p->migration_pending != pending)) goto out; if (task_on_rq_queued(p)) @@ -2024,13 +2041,6 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask) __do_set_cpus_allowed(p, new_mask, 0); } -struct set_affinity_pending { - refcount_t refs; - struct completion done; - struct cpu_stop_work stop_work; - struct migration_arg arg; -}; - /* * This function is wildly self concurrent; here be dragons. *
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