Re: [PATCH RFC] Remove the cancel_delayed_work() call from scsi_put_command()

[Hannes Reinecke <hare@suse.de>]

On 05/23/2014 01:28 PM, Paolo Bonzini wrote:
> Il 23/05/2014 12:37, Bart Van Assche ha scritto:
>> On 05/23/14 11:24, Paolo Bonzini wrote:
>>> Il 23/05/2014 08:09, Hannes Reinecke ha scritto:
>>>>
>>>> And when freeing a command we absolutely need to make sure that
>>>> the workqueue is empty.
>>>> So calling cancel_delayed_work() was the obvious thing to do.
>>>
>>> You would need cancel_delayed_work_sync, but if it really
>>> happened that
>>> the work item is running, it would cause a double free.
>>>
>>>> I'd be fine with adding a WARN_ON(!list_empty(&cmd->abort_work))
>>>> here, however. This will clear up the intent of this statement.
>>>
>>> BUG_ON even, since you'd get badness from the double free anyway.
>>
>> Hello Paolo,
>>
>> Are you aware that Linus strongly prefers WARN_ON_ONCE() over
>> BUG_ON() ?
>> See e.g. https://lkml.org/lkml/2012/9/27/461 or
>> https://lkml.org/lkml/2014/4/28/657.
>
> Yes, I am and I even downgraded some KVM BUG_ONs recently.
>
> But in this case I think that memory corruption is going to happen
> anyway unless you consciously leak the Scsi_Cmnd * (because if you
> use WARN_ON, you also need to return early as Linus suggested in the
> second email).
>
> So the WARN_ON/BUG_ON choice here should not just consider what
> makes the problem easier to debug; hanging the machine before
> guaranteed badness seems to me like a good use for BUG_ON.
>
So this should work, right?

diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 88d46fe..53b8b94 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -334,7 +334,7 @@ void scsi_put_command(struct scsi_cmnd *cmd)
         list_del_init(&cmd->list);
         spin_unlock_irqrestore(&cmd->device->list_lock, flags);

-       cancel_delayed_work(&cmd->abort_work);
+       BUG_ON(delayed_work_pending(&cmd->abort_work));

         __scsi_put_command(cmd->device->host, cmd);
  }

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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