RE: [PATCH] ACPI: NFIT: fix a potential deadlock during NFIT teardown

[Dan Williams]

Vishal Verma wrote:
> Lockdep reports that acpi_nfit_shutdown() may deadlock against an
> opportune acpi_nfit_scrub(). acpi_nfit_scrub () is run from inside a
> 'work' and therefore has already acquired workqueue-internal locks. It
> also acquiires acpi_desc->init_mutex. acpi_nfit_shutdown() first
> acquires init_mutex, and was subsequently attempting to cancel any
> pending workqueue items. This reversed locking order causes a potential
> deadlock:
> 
>     ======================================================
>     WARNING: possible circular locking dependency detected
>     6.2.0-rc3 #116 Tainted: G           O     N
>     ------------------------------------------------------
>     libndctl/1958 is trying to acquire lock:
>     ffff888129b461c0 ((work_completion)(&(&acpi_desc->dwork)->work)){+.+.}-{0:0}, at: __flush_work+0x43/0x450
> 
>     but task is already holding lock:
>     ffff888129b460e8 (&acpi_desc->init_mutex){+.+.}-{3:3}, at: acpi_nfit_shutdown+0x87/0xd0 [nfit]
> 
>     which lock already depends on the new lock.
> 
>     ...
> 
>     Possible unsafe locking scenario:
> 
>           CPU0                    CPU1
>           ----                    ----
>      lock(&acpi_desc->init_mutex);
>                                   lock((work_completion)(&(&acpi_desc->dwork)->work));
>                                   lock(&acpi_desc->init_mutex);
>      lock((work_completion)(&(&acpi_desc->dwork)->work));
> 
>     *** DEADLOCK ***
> 
> Since the workqueue manipulation is protected by its own internal locking,
> the cancellation of pending work doesn't need to be done under
> acpi_desc->init_mutex. Move cancel_delayed_work_sync() outside the
> init_mutex to fix the deadlock. Any work that starts after
> acpi_nfit_shutdown() drops the lock will see ARS_CANCEL, and the
> cancel_delayed_work_sync() will safely flush it out.
> 
> Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> ---
>  drivers/acpi/nfit/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/nfit/core.c b/drivers/acpi/nfit/core.c
> index f1cc5ec6a3b6..4e48d6db05eb 100644
> --- a/drivers/acpi/nfit/core.c
> +++ b/drivers/acpi/nfit/core.c
> @@ -3297,8 +3297,8 @@ void acpi_nfit_shutdown(void *data)
>  
>  	mutex_lock(&acpi_desc->init_mutex);
>  	set_bit(ARS_CANCEL, &acpi_desc->scrub_flags);
> -	cancel_delayed_work_sync(&acpi_desc->dwork);
>  	mutex_unlock(&acpi_desc->init_mutex);
> +	cancel_delayed_work_sync(&acpi_desc->dwork);

Looks good, applied.