[PATCH v2] kthread: fix task state in kthread worker if being frozen

From: Chen Yu
Date: Tue Aug 27 2024 - 07:27:20 EST


When analyzing a kernel waring message, Peter pointed out that there is a race
condition when the kworker is being frozen and falls into try_to_freeze() with
TASK_INTERRUPTIBLE, which could trigger a might_sleep() warning in try_to_freeze().
Although the root cause is not related to freeze()[1], it is still worthy to fix
this issue ahead.

One possible race scenario:

CPU 0 CPU 1
----- -----

// kthread_worker_fn
set_current_state(TASK_INTERRUPTIBLE);
suspend_freeze_processes()
freeze_processes
static_branch_inc(&freezer_active);
freeze_kernel_threads
pm_nosig_freezing = true;
if (work) { //false
__set_current_state(TASK_RUNNING);

} else if (!freezing(current)) //false, been frozen

freezing():
if (static_branch_unlikely(&freezer_active))
if (pm_nosig_freezing)
return true;
schedule()
}

// state is still TASK_INTERRUPTIBLE
try_to_freeze()
might_sleep() <--- warning

Fix this by explicitly set the TASK_RUNNING before entering
try_to_freeze().

Link: https://lore.kernel.org/lkml/Zs2ZoAcUsZMX2B%2FI@chenyu5-mobl2/ [1]
Fixes: b56c0d8937e6 ("kthread: implement kthread_worker")
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
v1->v2: Describe the race condition in commit log and
refined the code. (Andrew Morton)
---
kernel/kthread.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index f7be976ff88a..db4ceb0f503c 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -845,8 +845,16 @@ int kthread_worker_fn(void *worker_ptr)
* event only cares about the address.
*/
trace_sched_kthread_work_execute_end(work, func);
- } else if (!freezing(current))
+ } else if (!freezing(current)) {
schedule();
+ } else {
+ /*
+ * Handle the case where the current remains
+ * TASK_INTERRUPTIBLE. try_to_freeze() expects
+ * the current to be TASK_RUNNING.
+ */
+ __set_current_state(TASK_RUNNING);
+ }

try_to_freeze();
cond_resched();
--
2.25.1