Date: Thu, 23 Oct 2014 14:38:09 -0400 From: John Baldwin <jhb@freebsd.org> To: Mateusz Guzik <mjg@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r273549 - head/sys/kern Message-ID: <201410231438.09220.jhb@freebsd.org> In-Reply-To: <201410231535.s9NFZlbn002656@svn.freebsd.org> References: <201410231535.s9NFZlbn002656@svn.freebsd.org>
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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
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