Date: Thu, 23 Oct 2014 16:32:52 -0400 From: John Baldwin <jhb@freebsd.org> To: Adrian Chadd <adrian@freebsd.org> Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, Edward Tomasz =?utf-8?q?Napiera=C5=82a?= <trasz@freebsd.org>, Mateusz Guzik <mjg@freebsd.org> Subject: Re: svn commit: r273549 - head/sys/kern Message-ID: <201410231632.53172.jhb@freebsd.org> In-Reply-To: <CAJ-Vmo=teX0ggxDkjErA%2B_TwddedOsyeYXmkXv5pgB44d7ikBA@mail.gmail.com> References: <201410231535.s9NFZlbn002656@svn.freebsd.org> <20E7F407-9DC5-4A59-BC1C-7B868C31EF52@FreeBSD.org> <CAJ-Vmo=teX0ggxDkjErA%2B_TwddedOsyeYXmkXv5pgB44d7ikBA@mail.gmail.com>
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On Thursday, October 23, 2014 3:39:39 pm Adrian Chadd wrote: > Please back this out; it looks like the lock is protecting sf_si. The followup fix should be fine. The lock does indeed protect sf_si, but the value can only transition from non-NULL to NULL at this point, so if it is == NULL without the lock, it is safe to assume it has already been cleared. > -adrian > > On 23 October 2014 11:45, Edward Tomasz Napierała <trasz@freebsd.org> wrote: > > Dnia 23 paź 2014 o godz. 20:38 John Baldwin <jhb@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(). > > > > > > -- John Baldwinhelp
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