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Add two missing fences with comments describing them. These were found by

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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.
This commit is contained in:
Jeff Roberson 2020-01-31 22:21:15 +00:00
parent 82952c4065
commit 915c367e8e
2 changed files with 23 additions and 5 deletions
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17
sys/kern/subr_smr.c
View File

@ -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;
/*
@ -242,14 +242,19 @@ 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.
@ -336,6 +341,12 @@ 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);
}

11
sys/sys/smr.h
View File

@ -69,11 +69,18 @@ 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));
}
/*