svn commit: r273549 - head/sys/kern

[John Baldwin]

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.

-- 
John Baldwin