softlockup: ensure the task has been switched out once

When we check if a task has been switched out since the last scan, we might
have a race condition on the following scenario:

- the task is freshly created and scheduled

- it puts its state to TASK_UNINTERRUPTIBLE and is not yet switched out

- check_hung_task() scans this task and will report a false positive because
  t->nvcsw + t->nivcsw == t->last_switch_count == 0

Add a check for such cases.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 0c924de58cb2..022a4927b785 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -72,7 +72,13 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 {
         unsigned long switch_count = t->nvcsw + t->nivcsw;
 
-        if (t->flags & PF_FROZEN)
+        /*
+         * 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.
+         */
+        if (unlikely(t->flags & PF_FROZEN || !switch_count))
                 return;
 
         if (switch_count != t->last_switch_count) {