[PATCH] async: access bottom half flags with qatomic_read

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Resolves: #2749
Resolves: #851
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 util/async.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/util/async.c b/util/async.c
index 2719c629ae9..a736d2cd0d0 100644
--- a/util/async.c
+++ b/util/async.c
@@ -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;
             }
         }
-- 
2.51.0