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From: Jeff Roberson <jeff@FreeBSD.org>
Date: Thu, 6 Feb 2020 20:51:46 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
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Subject: svn commit: r357641 - head/sys/kern
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X-List-Received-Date: Thu, 06 Feb 2020 20:51:46 -0000
Author: jeff
Date: Thu Feb 6 20:51:46 2020
New Revision: 357641
URL: https://svnweb.freebsd.org/changeset/base/357641
Log:
Fix a race in smr_advance() that could result in unnecessary poll calls.
This was relatively harmless but surprising to see in counters. The
race occurred when rd_seq was read after the goal was updated and we
incorrectly calculated the delta between them.
Reviewed by: rlibby
Differential Revision: https://reviews.freebsd.org/D23464
Modified:
head/sys/kern/subr_smr.c
Modified: head/sys/kern/subr_smr.c
==============================================================================
--- head/sys/kern/subr_smr.c Thu Feb 6 20:47:50 2020 (r357640)
+++ head/sys/kern/subr_smr.c Thu Feb 6 20:51:46 2020 (r357641)
@@ -160,7 +160,7 @@ static uma_zone_t smr_zone;
#define SMR_SEQ_INCR (UINT_MAX / 10000)
#define SMR_SEQ_INIT (UINT_MAX - 100000)
/* Force extra polls to test the integer overflow detection. */
-#define SMR_SEQ_MAX_DELTA (1000)
+#define SMR_SEQ_MAX_DELTA (SMR_SEQ_INCR * 32)
#define SMR_SEQ_MAX_ADVANCE SMR_SEQ_MAX_DELTA / 2
#endif
@@ -188,7 +188,7 @@ smr_seq_t
smr_advance(smr_t smr)
{
smr_shared_t s;
- smr_seq_t goal;
+ smr_seq_t goal, s_rd_seq;
/*
* It is illegal to enter while in an smr section.
@@ -203,12 +203,18 @@ smr_advance(smr_t smr)
atomic_thread_fence_rel();
/*
+ * Load the current read seq before incrementing the goal so
+ * we are guaranteed it is always < goal.
+ */
+ s = zpcpu_get(smr)->c_shared;
+ s_rd_seq = atomic_load_acq_int(&s->s_rd_seq);
+
+ /*
* Increment the shared write sequence by 2. Since it is
* initialized to 1 this means the only valid values are
* odd and an observed value of 0 in a particular CPU means
* it is not currently in a read section.
*/
- s = zpcpu_get(smr)->c_shared;
goal = atomic_fetchadd_int(&s->s_wr_seq, SMR_SEQ_INCR) + SMR_SEQ_INCR;
counter_u64_add(advance, 1);
@@ -217,7 +223,7 @@ smr_advance(smr_t smr)
* far ahead of the read sequence number. This keeps the
* wrap detecting arithmetic working in pathological cases.
*/
- if (goal - atomic_load_int(&s->s_rd_seq) >= SMR_SEQ_MAX_DELTA) {
+ if (SMR_SEQ_DELTA(goal, s_rd_seq) >= SMR_SEQ_MAX_DELTA) {
counter_u64_add(advance_wait, 1);
smr_wait(smr, goal - SMR_SEQ_MAX_ADVANCE);
}