From 741a76b350897604c48fb12beff1c9b77724dc96 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Mon, 30 Apr 2018 14:50:22 +0200 Subject: [PATCH] kthread, sched/wait: Fix kthread_parkme() wait-loop Gaurav reported a problem with __kthread_parkme() where a concurrent try_to_wake_up() could result in competing stores to ->state which, when the TASK_PARKED store got lost bad things would happen. The comment near set_current_state() actually mentions this competing store, but only mentions the case against TASK_RUNNING. This same store, with different timing, can happen against a subsequent !RUNNING store. This normally is not a problem, because as per that same comment, the !RUNNING state store is inside a condition based wait-loop: for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); if (!need_sleep) break; schedule(); } __set_current_state(TASK_RUNNING); If we loose the (first) TASK_UNINTERRUPTIBLE store to a previous (concurrent) wakeup, the schedule() will NO-OP and we'll go around the loop once more. The problem here is that the TASK_PARKED store is not inside the KTHREAD_SHOULD_PARK condition wait-loop. There is a genuine issue with sleeps that do not have a condition; this is addressed in a subsequent patch. Reported-by: Gaurav Kohli Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Oleg Nesterov Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Signed-off-by: Ingo Molnar --- kernel/kthread.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index cd50e99202b0..cbee858e5815 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -177,12 +177,13 @@ void *kthread_probe_data(struct task_struct *task) static void __kthread_parkme(struct kthread *self) { - __set_current_state(TASK_PARKED); - while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) { + for (;;) { + set_current_state(TASK_PARKED); + if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags)) + break; if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags)) complete(&self->parked); schedule(); - __set_current_state(TASK_PARKED); } clear_bit(KTHREAD_IS_PARKED, &self->flags); __set_current_state(TASK_RUNNING);