Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/block-backend.c          | 4 ++--
 block/throttle-groups.c        | 2 +-
 include/sysemu/block-backend.h | 3 ++-
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 7405024..915ccc5 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1927,7 +1927,7 @@ static void blk_root_drained_begin(BdrvChild *child)
     /* Note that blk->root may not be accessible here yet if we are just
      * attaching to a BlockDriverState that is drained. Use child instead. */
 
-    if (blk->public.io_limits_disabled++ == 0) {
+    if (atomic_fetch_inc(&blk->public.io_limits_disabled) == 0) {
         throttle_group_restart_blk(blk);
     }
 }
@@ -1938,7 +1938,7 @@ static void blk_root_drained_end(BdrvChild *child)
     assert(blk->quiesce_counter);
 
     assert(blk->public.io_limits_disabled);
-    --blk->public.io_limits_disabled;
+    atomic_dec(&blk->public.io_limits_disabled);
 
     if (--blk->quiesce_counter == 0) {
         if (blk->dev_ops && blk->dev_ops->drained_end) {
diff --git a/block/throttle-groups.c b/block/throttle-groups.c
index b73e7a8..69bfbd4 100644
--- a/block/throttle-groups.c
+++ b/block/throttle-groups.c
@@ -240,7 +240,7 @@ static bool throttle_group_schedule_timer(BlockBackend *blk, bool is_write)
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     bool must_wait;
 
-    if (blkp->io_limits_disabled) {
+    if (atomic_read(&blkp->io_limits_disabled)) {
         return false;
     }
 
diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 7462228..87a43b0 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -80,7 +80,8 @@ typedef struct BlockBackendPublic {
     CoQueue      throttled_reqs[2];
 
     /* Nonzero if the I/O limits are currently being ignored; generally
-     * it is zero.  */
+     * it is zero.  Accessed with atomic operations.
+     */
     unsigned int io_limits_disabled;
 
     /* The following fields are protected by the ThrottleGroup lock.
-- 
2.9.3

On Thu, Apr 20, 2017 at 02:00:44PM +0200, Paolo Bonzini wrote:
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/block-backend.c          | 4 ++--
>  block/throttle-groups.c        | 2 +-
>  include/sysemu/block-backend.h | 3 ++-
>  3 files changed, 5 insertions(+), 4 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>