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

[Linux-MTD Mailing List]

Gitweb:     http://git.infradead.org/?p=mtd-2.6.git;a=commit;h=741a76b350897604c48fb12beff1c9b77724dc96
Commit:     741a76b350897604c48fb12beff1c9b77724dc96
Parent:     457be908c83637ee10bda085a23dc05afa3b14a0
Author:     Peter Zijlstra <peterz at infradead.org>
AuthorDate: Mon Apr 30 14:50:22 2018 +0200
Committer:  Ingo Molnar <mingo at kernel.org>
CommitDate: Thu May 3 07:38:04 2018 +0200

    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 <gkohli at codeaurora.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
    Reviewed-by: Oleg Nesterov <oleg at redhat.com>
    Cc: Linus Torvalds <torvalds at linux-foundation.org>
    Cc: Peter Zijlstra <peterz at infradead.org>
    Cc: Thomas Gleixner <tglx at linutronix.de>
    Signed-off-by: Ingo Molnar <mingo at kernel.org>
---
 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);