Re: [PATCH v2] driver core: Fix concurrent problem of deferred_probe_extend_timeout()

[Greg KH]

On Thu, Aug 14, 2025 at 08:29:49PM +0800, Wang Wensheng wrote:
> The deferred_probe_timeout_work may be canceled forever unexpected when
> deferred_probe_extend_timeout() executes concurrently. Start with
> deferred_probe_timeout_work pending, and the problem would
> occur after the following sequence.
> 
>          CPU0                                 CPU1
> deferred_probe_extend_timeout
>   -> cancel_delayed_work => true
>                                      deferred_probe_extend_timeout
>                                        -> cancel_delayed_wrok
>                                          -> __cancel_work
>                                            -> try_grab_pending
>   -> schedule_delayed_work
>    -> queue_delayed_work_on
> since pending bit is grabbed,
> just return without doing anything
>                                         -> set_work_pool_and_clear_pending
>                                      this __cancel_work return false and
>                                      the work would never be queued again
> 
> The root cause is that the PENDING_BIT of the work_struct would be set
> temporaily in __cancel_work and this bit could prevent the work_struct
> to be queued in another CPU.
> 
> Use deferred_probe_mutex to protect the cancel and queue operations for
> the deferred_probe_timeout_work to fix this problem.
> 
> Fixes: 2b28a1a84a0e ("driver core: Extend deferred probe timeout on driver registration")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Wang Wensheng <wangwensheng4@xxxxxxxxxx>
> ---
>  drivers/base/dd.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 13ab98e033ea..00419d2ee910 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -323,6 +323,7 @@ static DECLARE_DELAYED_WORK(deferred_probe_timeout_work, deferred_probe_timeout_
>  
>  void deferred_probe_extend_timeout(void)
>  {
> +	guard(mutex)(&deferred_probe_mutex);

But if you grab the lock here, in the probe timeout function, the lock
will be grabbed again, causing a deadlock, right?  If not, why not?

Have you run this patch with lockdep enabled?

This feels broken to me, what am I missing?

thanks,

greg k-h