Re: [PATCH 2/2] drivers/usb: Delete XHCI command timer if necessary

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 11.08.2015 11:15, Oliver Neukum wrote:
> On Mon, 2015-08-03 at 16:07 +0300, Mathias Nyman wrote:
>> From: Gavin Shan <gwshan@linux.vnet.ibm.com>
>>
>> When xhci_mem_cleanup() is called, it's possible that the command
>> timer isn't initialized and scheduled. For those cases, to delete
>> the command timer causes soft-lockup as below stack dump shows.
>>
>> The patch avoids deleting the command timer if it's not scheduled
>> with the help of timer_pending().
> 
> Are you sure this is safe? timer_pending() will not show you that
> the timer function is running. It looks like you introduced a race
> between timeout and cleanup.
> 

Looking at it in more detail you're right.

Fortunately this can only happen in cases where xhci is already hosed
(no command response for 5 seconds), and we are at the same time
anyway about to remove xhci.

Doesn't this mean that all cases with
if (timer_pending(&timer))
	del_timer_sync(&timer)

is just basically the same as a plain del_timer(&timer)?

Anyways, turns out that the error path in xhci initialization code can end up calling
del_timer_sync() before timer is initialized. This should be fixed by re-arranging
some code in xhci initialization instead.

Greg, should this be reverted in rc7?
I think that the possible side effect of this patch is still lesser the original
issue.     

Thanks for spotting this

-Mathias