svn commit: r273549 - head/sys/kern

[Adrian Chadd]

Please back this out; it looks like the lock is protecting sf_si.


-adrian

On 23 October 2014 11:45, Edward Tomasz Napierała <trasz at freebsd.org> wrote:
> Dnia 23 paź 2014 o godz. 20:38 John Baldwin <jhb at freebsd.org> napisał(a):
>
>>> On Thursday, October 23, 2014 11:35:47 am Mateusz Guzik wrote:
>>> Author: mjg
>>> Date: Thu Oct 23 15:35:47 2014
>>> New Revision: 273549
>>> URL: https://svnweb.freebsd.org/changeset/base/273549
>>>
>>> Log:
>>>  Avoid taking the lock in selfdfree when not needed.
>>>
>>> Modified:
>>>  head/sys/kern/sys_generic.c
>>>
>>> Modified: head/sys/kern/sys_generic.c
>> ==============================================================================
>>> --- head/sys/kern/sys_generic.c    Thu Oct 23 15:16:40 2014    (r273548)
>>> +++ head/sys/kern/sys_generic.c    Thu Oct 23 15:35:47 2014    (r273549)
>>> @@ -1600,10 +1600,11 @@ static void
>>> selfdfree(struct seltd *stp, struct selfd *sfp)
>>> {
>>>    STAILQ_REMOVE(&stp->st_selq, sfp, selfd, sf_link);
>>> -    mtx_lock(sfp->sf_mtx);
>>> -    if (sfp->sf_si)
>>> +    if (sfp->sf_si != NULL) {
>>> +        mtx_lock(sfp->sf_mtx);
>>>        TAILQ_REMOVE(&sfp->sf_si->si_tdlist, sfp, sf_threads);
>>> -    mtx_unlock(sfp->sf_mtx);
>>> +        mtx_unlock(sfp->sf_mtx);
>>> +    }
>>>    uma_zfree(selfd_zone, sfp);
>>
>> How do you ensure that the value you read for sf_si here is up to date?  In
>> particular, if a thread is selecting on multiple fds and one awakens it,
>> another fd can invoke selwakeup() while the thread is in seltdclear().
>> In that case, you might see a stale value of sf_si and not realize it is
>> cleared by the selwakeup() after you get the lock and you will invoke
>> TAILQ_REMOVE an extra time.
>
> FWIW, I've just hit a panic in selfdfree().
>
>