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Index: sys/kern/subr_smr.c
===================================================================
--- sys/kern/subr_smr.c
+++ sys/kern/subr_smr.c
@@ -188,7 +188,7 @@
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.
@@ -202,13 +202,19 @@
*/
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 @@
* 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);
}
Index: sys/sys/smr.h
===================================================================
--- sys/sys/smr.h
+++ sys/sys/smr.h
@@ -49,6 +49,7 @@
#define SMR_SEQ_LEQ(a, b) ((int32_t)((a)-(b)) <= 0)
#define SMR_SEQ_GT(a, b) ((int32_t)((a)-(b)) > 0)
#define SMR_SEQ_GEQ(a, b) ((int32_t)((a)-(b)) >= 0)
+#define SMR_SEQ_DELTA(a, b) ((int32_t)((a)-(b)))
#define SMR_SEQ_INVALID 0

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