[76/80] hung_task: fix false positive during vfork

[Greg KH]

3.1-stable review patch.  If anyone has any objections, please let me know.

------------------
Content-Length: 1649
Lines: 48

From: Mandeep Singh Baines <msb@xxxxxxxxxxxx>

commit f9fab10bbd768b0e5254e53a4a8477a94bfc4b96 upstream.

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@xxxxxxxxxxxx>
Reported-by: Sasha Levin <levinsasha928@xxxxxxxxx>
LKML-Reference: <1325344394.28904.43.camel@lappy>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: John Kacur <jkacur@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 kernel/hung_task.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -74,11 +74,17 @@ static void check_hung_task(struct task_
 
 	/*
 	 * 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) {


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