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) {
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);
}
/* 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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