This is a note to let you know that I've just added the patch titled
workqueue: Fix spruious data race in __flush_work()
to the 6.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
workqueue-fix-spruious-data-race-in-__flush_work.patch
and it can be found in the queue-6.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit aa9797bef1f118706e73b6719191b83e35e696e2
Author: Tejun Heo <tj@xxxxxxxxxx>
Date: Mon Aug 5 09:37:25 2024 -1000
workqueue: Fix spruious data race in __flush_work()
[ Upstream commit 8bc35475ef1a23b0e224f3242eb11c76cab0ea88 ]
When flushing a work item for cancellation, __flush_work() knows that it
exclusively owns the work item through its PENDING bit. 134874e2eee9
("workqueue: Allow cancel_work_sync() and disable_work() from atomic
contexts on BH work items") added a read of @work->data to determine whether
to use busy wait for BH work items that are being canceled. While the read
is safe when @from_cancel, @work->data was read before testing @from_cancel
to simplify code structure:
data = *work_data_bits(work);
if (from_cancel &&
!WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && (data & WORK_OFFQ_BH)) {
While the read data was never used if !@from_cancel, this could trigger
KCSAN data race detection spuriously:
==================================================================
BUG: KCSAN: data-race in __flush_work / __flush_work
write to 0xffff8881223aa3e8 of 8 bytes by task 3998 on cpu 0:
instrument_write include/linux/instrumented.h:41 [inline]
___set_bit include/asm-generic/bitops/instrumented-non-atomic.h:28 [inline]
insert_wq_barrier kernel/workqueue.c:3790 [inline]
start_flush_work kernel/workqueue.c:4142 [inline]
__flush_work+0x30b/0x570 kernel/workqueue.c:4178
flush_work kernel/workqueue.c:4229 [inline]
...
read to 0xffff8881223aa3e8 of 8 bytes by task 50 on cpu 1:
__flush_work+0x42a/0x570 kernel/workqueue.c:4188
flush_work kernel/workqueue.c:4229 [inline]
flush_delayed_work+0x66/0x70 kernel/workqueue.c:4251
...
value changed: 0x0000000000400000 -> 0xffff88810006c00d
Reorganize the code so that @from_cancel is tested before @work->data is
accessed. The only problem is triggering KCSAN detection spuriously. This
shouldn't need READ_ONCE() or other access qualifiers.
No functional changes.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: syzbot+b3e4f2f51ed645fd5df2@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 134874e2eee9 ("workqueue: Allow cancel_work_sync() and disable_work() from atomic contexts on BH work items")
Link: http://lkml.kernel.org/r/000000000000ae429e061eea2157@xxxxxxxxxx
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c8687c0ab3645..c970eec25c5a0 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4190,7 +4190,6 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
static bool __flush_work(struct work_struct *work, bool from_cancel)
{
struct wq_barrier barr;
- unsigned long data;
if (WARN_ON(!wq_online))
return false;
@@ -4208,29 +4207,35 @@ static bool __flush_work(struct work_struct *work, bool from_cancel)
* was queued on a BH workqueue, we also know that it was running in the
* BH context and thus can be busy-waited.
*/
- data = *work_data_bits(work);
- if (from_cancel &&
- !WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && (data & WORK_OFFQ_BH)) {
- /*
- * On RT, prevent a live lock when %current preempted soft
- * interrupt processing or prevents ksoftirqd from running by
- * keeping flipping BH. If the BH work item runs on a different
- * CPU then this has no effect other than doing the BH
- * disable/enable dance for nothing. This is copied from
- * kernel/softirq.c::tasklet_unlock_spin_wait().
- */
- while (!try_wait_for_completion(&barr.done)) {
- if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
- local_bh_disable();
- local_bh_enable();
- } else {
- cpu_relax();
+ if (from_cancel) {
+ unsigned long data = *work_data_bits(work);
+
+ if (!WARN_ON_ONCE(data & WORK_STRUCT_PWQ) &&
+ (data & WORK_OFFQ_BH)) {
+ /*
+ * On RT, prevent a live lock when %current preempted
+ * soft interrupt processing or prevents ksoftirqd from
+ * running by keeping flipping BH. If the BH work item
+ * runs on a different CPU then this has no effect other
+ * than doing the BH disable/enable dance for nothing.
+ * This is copied from
+ * kernel/softirq.c::tasklet_unlock_spin_wait().
+ */
+ while (!try_wait_for_completion(&barr.done)) {
+ if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+ local_bh_disable();
+ local_bh_enable();
+ } else {
+ cpu_relax();
+ }
}
+ goto out_destroy;
}
- } else {
- wait_for_completion(&barr.done);
}
+ wait_for_completion(&barr.done);
+
+out_destroy:
destroy_work_on_stack(&barr.work);
return true;
}
