kthread, sched/wait: Fix kthread_parkme() wait-loop

BugLink: http://bugs.launchpad.net/bugs/1794889
[ Upstream commit 741a76b350897604c48fb12beff1c9b77724dc96 ]

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 <gkohli@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 22c1fc77848ad2f3ffd3fd32c44a605907790a68..9ea47128dbd04362ca71552f222ce073fb6ee4d5 100644 (file)
--- 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);