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Replace non-loop usage of atomic_compare_exchange_weak with strong variant

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While testing BIND 9 on arm64 8+ core machine, it was discovered that
the weak variants in fact does spuriously fail, we haven't observed that
on other architectures.

This commit replaces all non-loop usage of atomic_compare_exchange_weak
with atomic_compare_exchange_strong.
This commit is contained in:
Ondřej Surý
2020-02-12 09:17:55 +01:00
parent fa68a0d869
commit 4cf275ba8a
2 changed files with 36 additions and 50 deletions
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52
lib/isc/rwlock.c
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@@ -400,14 +400,10 @@ isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
break;
}
while (1) {
int_fast32_t zero = 0;
if (atomic_compare_exchange_weak_acq_rel(
&rwl->cnt_and_flag, &zero, WRITER_ACTIVE))
{
break;
}
while (!atomic_compare_exchange_weak_acq_rel(
&rwl->cnt_and_flag, &(int_fast32_t){ 0 },
WRITER_ACTIVE))
{
/* Another active reader or writer is working. */
LOCK(&rwl->lock);
if (atomic_load_acquire(&rwl->cnt_and_flag) != 0) {
@@ -494,8 +490,8 @@ isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
} else {
/* Try locking without entering the waiting queue. */
int_fast32_t zero = 0;
if (!atomic_compare_exchange_weak_acq_rel(&rwl->cnt_and_flag,
&zero, WRITER_ACTIVE))
if (!atomic_compare_exchange_strong_acq_rel(
&rwl->cnt_and_flag, &zero, WRITER_ACTIVE))
{
return (ISC_R_LOCKBUSY);
}
@@ -519,28 +515,26 @@ isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
REQUIRE(VALID_RWLOCK(rwl));
{
int_fast32_t reader_incr = READER_INCR;
int_fast32_t reader_incr = READER_INCR;
/* Try to acquire write access. */
atomic_compare_exchange_weak_acq_rel(
&rwl->cnt_and_flag, &reader_incr, WRITER_ACTIVE);
/* Try to acquire write access. */
atomic_compare_exchange_strong_acq_rel(&rwl->cnt_and_flag, &reader_incr,
WRITER_ACTIVE);
/*
* There must have been no writer, and there must have
* been at least one reader.
*/
INSIST((reader_incr & WRITER_ACTIVE) == 0 &&
(reader_incr & ~WRITER_ACTIVE) != 0);
if (reader_incr == READER_INCR) {
/*
* There must have been no writer, and there must have
* been at least one reader.
* We are the only reader and have been upgraded.
* Now jump into the head of the writer waiting queue.
*/
INSIST((reader_incr & WRITER_ACTIVE) == 0 &&
(reader_incr & ~WRITER_ACTIVE) != 0);
if (reader_incr == READER_INCR) {
/*
* We are the only reader and have been upgraded.
* Now jump into the head of the writer waiting queue.
*/
atomic_fetch_sub_release(&rwl->write_completions, 1);
} else {
return (ISC_R_LOCKBUSY);
}
atomic_fetch_sub_release(&rwl->write_completions, 1);
} else {
return (ISC_R_LOCKBUSY);
}
return (ISC_R_SUCCESS);