From: Valentin Schneider valentin.schneider@arm.com
[ Upstream commit 475ea6c60279e9f2ddf7e4cf2648cd8ae0608361 ]
Will reported that the 'XXX __migrate_task() can fail' in migration_cpu_stop() can happen, and it *is* sort of a big deal. Looking at it some more, one will note there is a glaring hole in the deferred CPU selection:
(w/ CONFIG_CPUSET=n, so that the affinity mask passed via taskset doesn't get AND'd with cpu_online_mask)
$ taskset -pc 0-2 $PID # offline CPUs 3-4 $ taskset -pc 3-5 $PID `\ $PID may stay on 0-2 due to the cpumask_any_distribute() picking an offline CPU and __migrate_task() refusing to do anything due to cpu_is_allowed().
set_cpus_allowed_ptr() goes to some length to pick a dest_cpu that matches the right constraints vs affinity and the online/active state of the CPUs. Reuse that instead of discarding it in the affine_move_task() case.
Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()") Reported-by: Will Deacon will@kernel.org Signed-off-by: Valentin Schneider valentin.schneider@arm.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/20210526205751.842360-2-valentin.schneider@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/core.c | 20 ++++++++++++-------- 1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 94002cb551c4..6e0ebc0781d1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1929,7 +1929,6 @@ static int migration_cpu_stop(void *data) struct migration_arg *arg = data; struct set_affinity_pending *pending = arg->pending; struct task_struct *p = arg->task; - int dest_cpu = arg->dest_cpu; struct rq *rq = this_rq(); bool complete = false; struct rq_flags rf; @@ -1967,19 +1966,15 @@ static int migration_cpu_stop(void *data) if (pending) { p->migration_pending = NULL; complete = true; - }
- if (dest_cpu < 0) { if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) goto out; - - dest_cpu = cpumask_any_distribute(&p->cpus_mask); }
if (task_on_rq_queued(p)) - rq = __migrate_task(rq, &rf, p, dest_cpu); + rq = __migrate_task(rq, &rf, p, arg->dest_cpu); else - p->wake_cpu = dest_cpu; + p->wake_cpu = arg->dest_cpu;
/* * XXX __migrate_task() can fail, at which point we might end @@ -2262,7 +2257,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag init_completion(&my_pending.done); my_pending.arg = (struct migration_arg) { .task = p, - .dest_cpu = -1, /* any */ + .dest_cpu = dest_cpu, .pending = &my_pending, };
@@ -2270,6 +2265,15 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag } else { pending = p->migration_pending; refcount_inc(&pending->refs); + /* + * Affinity has changed, but we've already installed a + * pending. migration_cpu_stop() *must* see this, else + * we risk a completion of the pending despite having a + * task on a disallowed CPU. + * + * Serialized by p->pi_lock, so this is safe. + */ + pending->arg.dest_cpu = dest_cpu; } } pending = p->migration_pending;