hung_task: fix false positive during vfork

vfork parent uninterruptibly and unkillably waits for its child to
exec/exit. This wait is of unbounded length. Ignore such waits
in the hung_task detector.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Reported-by: Sasha Levin <levinsasha928@gmail.com>
LKML-Reference: <1325344394.28904.43.camel@lappy>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Kacur <jkacur@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 4 deletions

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 8b1748d0172c..2e48ec0c2e91 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -74,11 +74,17 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 
         /*
          * Ensure the task is not frozen.
-         * Also, when a freshly created task is scheduled once, changes
-         * its state to TASK_UNINTERRUPTIBLE without having ever been
-         * switched out once, it musn't be checked.
+         * Also, skip vfork and any other user process that freezer should skip.
          */
-        if (unlikely(t->flags & PF_FROZEN || !switch_count))
+        if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
+            return;
+
+        /*
+         * When a freshly created task is scheduled once, changes its state to
+         * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
+         * musn't be checked.
+         */
+        if (unlikely(!switch_count))
                 return;
 
         if (switch_count != t->last_switch_count) {