Re: [syzbot] WARNING in call_timer_fn

[Tetsuo Handa]

On 2022/11/18 6:16, Luiz Augusto von Dentz wrote:
> Wasn't the following patch suppose to address such problem:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth-next.git/commit/?id=deee93d13d385103205879a8a0915036ecd83261
> 
> It was merged in the last pull request to net-next:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth-next.git/commit/?id=a507ea32b9c2c407012bf89109ac0cf89fae313c
> 

No. Commit deee93d13d38 ("Bluetooth: use hdev->workqueue when queuing hdev->{cmd,ncmd}_timer works")
is for handling queue_work() from hci_cmd_timeout(struct work_struct *work) from process_one_work()
(that is, called from a kernel workqueue thread from process context).

But this report says that queue_work() is called from timer interrupt handler from interrupt context
while drain_workqueue(hdev->workqueue) is in progress from process context.

But... is is_chained_work() check appropriate?
Why can't we exclude "timer interrupt handler" from "somebody else" ?

The comment for drain_workqueue() says

 * Wait until the workqueue becomes empty.  While draining is in progress,
 * only chain queueing is allowed.  IOW, only currently pending or running
 * work items on @wq can queue further work items on it.  @wq is flushed
 * repeatedly until it becomes empty.  The number of flushing is determined
 * by the depth of chaining and should be relatively short.  Whine if it
 * takes too long.

but why limited to "only currently pending or running work items on @wq" (that is,
only process context) ?

Although drain_workqueue() is also called from destroy_workqueue() (which would cause
use-after-free bug if interrupt handler calls queue_work() some time after drain_workqueue()),
I think that we can wait for drain_workqueue() to call __flush_workqueue() again if a further
work item is queued from interrupt handler...

Anyway, stopping all works and delayed works before calling drain_workqueue() would address
this problem.