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Fix a race in smr_advance() that could result in unnecessary poll calls.

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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
This commit is contained in:
Jeff Roberson 2020-02-06 20:51:46 +00:00
parent cd0be8b2ed
commit a40068e524
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=357641
1 changed files with 10 additions and 4 deletions
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14
sys/kern/subr_smr.c
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@ -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.
@ -202,13 +202,19 @@ 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);
}