From cf2592f59c0e8ed4308adbdb2e0a88655379d579 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Tue, 10 Feb 2009 16:52:37 +0100
Subject: [PATCH] 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>
---
kernel/hung_task.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
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) {