Date: Tue, 17 Nov 2020 03:58:25 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjg@freebsd.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r367713 - head/sys/kern Message-ID: <X7MuQVV49j7ynzdN@kib.kiev.ua> In-Reply-To: <202011160309.0AG39JmP067464@repo.freebsd.org> References: <202011160309.0AG39JmP067464@repo.freebsd.org>
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On Mon, Nov 16, 2020 at 03:09:19AM +0000, Mateusz Guzik wrote: > Author: mjg > Date: Mon Nov 16 03:09:18 2020 > New Revision: 367713 > URL: https://svnweb.freebsd.org/changeset/base/367713 > > Log: > select: replace reference counting with memory barriers in selfd > > Refcounting was added to combat a race between selfdfree and doselwakup, > but it adds avoidable overhead. > > selfdfree detects it can free the object by ->sf_si == NULL, thus we can > ensure that the condition only holds after all accesses are completed. > > Modified: > head/sys/kern/sys_generic.c > > Modified: head/sys/kern/sys_generic.c > ============================================================================== > --- head/sys/kern/sys_generic.c Sun Nov 15 22:49:28 2020 (r367712) > +++ head/sys/kern/sys_generic.c Mon Nov 16 03:09:18 2020 (r367713) > @@ -156,7 +156,6 @@ struct selfd { > struct mtx *sf_mtx; /* Pointer to selinfo mtx. */ > struct seltd *sf_td; /* (k) owning seltd. */ > void *sf_cookie; /* (k) fd or pollfd. */ > - u_int sf_refs; > }; > > MALLOC_DEFINE(M_SELFD, "selfd", "selfd"); > @@ -1704,16 +1703,17 @@ static void > selfdfree(struct seltd *stp, struct selfd *sfp) > { > STAILQ_REMOVE(&stp->st_selq, sfp, selfd, sf_link); > - if (sfp->sf_si != NULL) { > + /* > + * Paired with doselwakeup. > + */ > + if (atomic_load_acq_ptr((uintptr_t *)&sfp->sf_si) != (uintptr_t)NULL) { This could be != 0. > mtx_lock(sfp->sf_mtx); > if (sfp->sf_si != NULL) { > TAILQ_REMOVE(&sfp->sf_si->si_tdlist, sfp, sf_threads); > - refcount_release(&sfp->sf_refs); > } > mtx_unlock(sfp->sf_mtx); > } > - if (refcount_release(&sfp->sf_refs)) > - free(sfp, M_SELFD); > + free(sfp, M_SELFD); What guarantees that doselwakeup() finished with sfp ? > } > > /* Drain the waiters tied to all the selfd belonging the specified selinfo. */ > @@ -1766,7 +1766,6 @@ selrecord(struct thread *selector, struct selinfo *sip > */ > sfp->sf_si = sip; > sfp->sf_mtx = mtxp; > - refcount_init(&sfp->sf_refs, 2); > STAILQ_INSERT_TAIL(&stp->st_selq, sfp, sf_link); > /* > * Now that we've locked the sip, check for initialization. > @@ -1820,14 +1819,15 @@ doselwakeup(struct selinfo *sip, int pri) > * sf_si seltdclear will know to ignore this si. > */ > TAILQ_REMOVE(&sip->si_tdlist, sfp, sf_threads); > - sfp->sf_si = NULL; > stp = sfp->sf_td; > + /* > + * Paired with selfdfree. > + */ > + atomic_store_rel_ptr((uintptr_t *)&sfp->sf_si, (uintptr_t)NULL); > mtx_lock(&stp->st_mtx); > stp->st_flags |= SELTD_PENDING; > cv_broadcastpri(&stp->st_wait, pri); > mtx_unlock(&stp->st_mtx); > - if (refcount_release(&sfp->sf_refs)) > - free(sfp, M_SELFD); > } > mtx_unlock(sip->si_mtx); > }
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