On 11/17/20, Konstantin Belousov <kostik...@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. -- Mateusz Guzik <mjguzik gmail.com> _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
