Date: Tue, 17 Nov 2020 05:20:51 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com> 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: <X7NBkwpj9vxm5nvm@kib.kiev.ua> In-Reply-To: <CAGudoHFh2MBNAg26fVzud-rgd5qPVZi7G-mM-dB-p8ba5=gHwQ@mail.gmail.com> References: <202011160309.0AG39JmP067464@repo.freebsd.org> <X7MuQVV49j7ynzdN@kib.kiev.ua> <CAGudoHFh2MBNAg26fVzud-rgd5qPVZi7G-mM-dB-p8ba5=gHwQ@mail.gmail.com>
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On Tue, Nov 17, 2020 at 04:15:12AM +0100, Mateusz Guzik wrote: > On 11/17/20, Konstantin Belousov <kostikbel@gmail.com> wrote: > > 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 ? > > > > Release semantics provided by atomic_store_rel_ptr -- it means the > NULL store is the last access, neither CPU nor the compiler are going > to reorder preceding loads/stores past it. It only guarantees that if we observed NULL as the result of load.
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