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workqueue: Fix spruious data race in __flush_work()

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When flushing a work item for cancellation, __flush_work() knows that it
exclusively owns the work item through its PENDING bit. 134874e2ee
("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@kernel.org>
Reported-by: syzbot+b3e4f2f51ed645fd5df2@syzkaller.appspotmail.com
Fixes: 134874e2ee ("workqueue: Allow cancel_work_sync() and disable_work() from atomic contexts on BH work items")
Link: http://lkml.kernel.org/r/000000000000ae429e061eea2157@google.com
Cc: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Tejun Heo 2024-08-05 09:37:25 -10:00
parent 98cc1730c8
commit 8bc35475ef
1 changed files with 25 additions and 20 deletions
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45
kernel/workqueue.c
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@ -4166,7 +4166,6 @@ already_gone:
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;
@ -4184,29 +4183,35 @@ 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;
}