Author: jeff
Date: Fri Jan 31 22:21:15 2020
New Revision: 357355
URL: https://svnweb.freebsd.org/changeset/base/357355
Log:
Add two missing fences with comments describing them. These were found by
inspection and after a lengthy discussion with jhb and kib. They have not
produced test failures.
Don't pointer chase through cpu0's smr. Use cpu correct smr even when not
in a critical section to reduce the likelihood of false sharing.
Modified:
head/sys/kern/subr_smr.c
head/sys/sys/smr.h
Modified: head/sys/kern/subr_smr.c
==============================================================================
--- head/sys/kern/subr_smr.c Fri Jan 31 21:20:22 2020 (r357354)
+++ head/sys/kern/subr_smr.c Fri Jan 31 22:21:15 2020 (r357355)
@@ -195,7 +195,7 @@ smr_advance(smr_t smr)
* odd and an observed value of 0 in a particular CPU means
* it is not currently in a read section.
*/
- s = smr->c_shared;
+ s = zpcpu_get(smr)->c_shared;
goal = atomic_fetchadd_int(&s->s_wr_seq, SMR_SEQ_INCR) + SMR_SEQ_INCR;
/*
@@ -242,16 +242,21 @@ smr_poll(smr_t smr, smr_seq_t goal, bool wait)
*/
success = true;
critical_enter();
- s = smr->c_shared;
+ s = zpcpu_get(smr)->c_shared;
/*
* Acquire barrier loads s_wr_seq after s_rd_seq so that we can not
* observe an updated read sequence that is larger than write.
*/
s_rd_seq = atomic_load_acq_int(&s->s_rd_seq);
- s_wr_seq = smr_current(smr);
/*
+ * wr_seq must be loaded prior to any c_seq value so that a stale
+ * c_seq can only reference time after this wr_seq.
+ */
+ s_wr_seq = atomic_load_acq_int(&s->s_wr_seq);
+
+ /*
* Detect whether the goal is valid and has already been observed.
*
* The goal must be in the range of s_wr_seq >= goal >= s_rd_seq for
@@ -335,6 +340,12 @@ smr_poll(smr_t smr, smr_seq_t goal, bool wait)
out:
critical_exit();
+
+ /*
+ * Serialize with smr_advance()/smr_exit(). The caller is now free
+ * to modify memory as expected.
+ */
+ atomic_thread_fence_acq();
return (success);
}
Modified: head/sys/sys/smr.h
==============================================================================
--- head/sys/sys/smr.h Fri Jan 31 21:20:22 2020 (r357354)
+++ head/sys/sys/smr.h Fri Jan 31 22:21:15 2020 (r357355)
@@ -70,10 +70,17 @@ struct smr {
* Return the current write sequence number.
*/
static inline smr_seq_t
+smr_shared_current(smr_shared_t s)
+{
+
+ return (atomic_load_int(&s->s_wr_seq));
+}
+
+static inline smr_seq_t
smr_current(smr_t smr)
{
- return (atomic_load_int(&smr->c_shared->s_wr_seq));
+ return (smr_shared_current(zpcpu_get(smr)->c_shared));
}
/*
@@ -106,7 +113,7 @@ smr_enter(smr_t smr)
* is detected and handled there.
*/
/* This is an add because we do not have atomic_store_acq_int */
- atomic_add_acq_int(&smr->c_seq, smr_current(smr));
+ atomic_add_acq_int(&smr->c_seq, smr_shared_current(smr->c_shared));
}
/*
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