Re: [PATCH 0/2] Race fixes in sdhci

[Chris Ball <cjb@laptop.org>]

Hi,

On Wed, Apr 27 2011, Andrei Warkentin wrote:
> On Wed, Apr 27, 2011 at 8:23 AM, Mark Brown
> <broonie@opensource.wolfsonmicro.com> wrote:
>> I've had this pair of patches sitting in my tree for a while now (I
>> believe they were previously posted) providing stability improvements in
>> sdhci on my systems.  Having looked through the code I believe but have
>> not confirmed that the issue is that the timeout is racing with an
>> actual completion of a pending task - both paths will trigger the
>> tasklet, and if you trigger a tasklet while it is running this causes it
>> to be rescheduled.  The result will be that the tasklet gets run a
>> second time with no work pending for it.
>>
>> I'm not convinced that these are the best fixes (it feels like we should
>> instead be closing the races down) but I don't really have time to come
>> up with something better myself right now so I'm pushing them out as-is
>> for comment.
>
> So the timeout interrupt occurs after even though the command
> succeeds? Am I interpreting that correctly?

No, I think Mark's saying there's a race of:

* the successful completion interrupt fires, and

* the host timer fires to signify timeout due to *lack* of an interrupt
  (via sdhci_timeout_timer()).  i.e., the completion interrupt fires
  very close to the timeout period.

Both cases call tasklet_schedule(&host->finish_tasklet), and if you
manage to schedule a tasklet while it's already running, it runs again
after it completes -- but during the first run we set host->mrq->cmd to
NULL, so then it oopses on the second run.

We could consider taking Mark's first patch and also adding to the top:

        /*
         * If we get scheduled twice concurrently, this tasklet will
         * be run again afterwards but without any active request.
         */
        if (!host->mrq)
                return;

.. and pushing to .39 with a stable@ tag.

- Chris.
-- 
Chris Ball   <cjb@laptop.org>   <http://printf.net/>
One Laptop Per Child