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async: access bottom half flags with qatomic_read

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Running test-aio-multithread under TSAN reveals data races on bh->flags.
Because bottom halves may be scheduled or canceled asynchronously,
without taking a lock, adjust aio_compute_bh_timeout() and aio_ctx_check()
to use a relaxed read to access the flags.

Use an acquire load to ensure that anything that was written prior to
qemu_bh_schedule() is visible.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/2749
Closes: https://gitlab.com/qemu-project/qemu/-/issues/851
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 5142397c79)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
This commit is contained in:
Paolo Bonzini
2025-10-13 18:24:54 +02:00
committed by Michael Tokarev
parent f316e90a82
commit 4e58c5bf0c
1 changed files with 7 additions and 4 deletions
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11
util/async.c
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@@ -256,8 +256,9 @@ static int64_t aio_compute_bh_timeout(BHList *head, int timeout)
QEMUBH *bh;
QSLIST_FOREACH_RCU(bh, head, next) {
if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
if (bh->flags & BH_IDLE) {
int flags = qatomic_load_acquire(&bh->flags);
if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
if (flags & BH_IDLE) {
/* idle bottom halves will be polled at least
* every 10ms */
timeout = 10000000;
@@ -335,14 +336,16 @@ aio_ctx_check(GSource *source)
aio_notify_accept(ctx);
QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {
if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
int flags = qatomic_load_acquire(&bh->flags);
if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
return true;
}
}
QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
QSLIST_FOREACH_RCU(bh, &s->bh_list, next) {
if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
int flags = qatomic_load_acquire(&bh->flags);
if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
return true;
}
}