block: never use atomics to access bs->quiesce_counter

All accesses of bs->quiesce_counter are in the main thread, either
after a GLOBAL_STATE_CODE() macro or in a function with GRAPH_WRLOCK
annotation.

This is essentially a revert of 414c2ec358 ("block: access
quiesce_counter with atomic ops"). At that time, neither the
GLOBAL_STATE_CODE() macro nor the GRAPH_WRLOCK annotation existed.
Even if the field was only accessed in the main thread back then (did
not check if that is actually the case), it wouldn't have been easy to
verify.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-24-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -361,7 +361,7 @@ static void bdrv_do_drained_begin(BlockDriverState *bs, BdrvChild *parent,
     GLOBAL_STATE_CODE();
 
     /* Stop things in parent-to-child order */
-    if (qatomic_fetch_inc(&bs->quiesce_counter) == 0) {
+    if (bs->quiesce_counter++ == 0) {
         GRAPH_RDLOCK_GUARD_MAINLOOP();
         bdrv_parent_drained_begin(bs, parent);
         if (bs->drv && bs->drv->bdrv_drain_begin) {
@@ -401,8 +401,6 @@ bdrv_drained_begin(BlockDriverState *bs)
  */
 static void bdrv_do_drained_end(BlockDriverState *bs, BdrvChild *parent)
 {
-    int old_quiesce_counter;
-
     IO_OR_GS_CODE();
 
     if (qemu_in_coroutine()) {
@@ -415,8 +413,7 @@ static void bdrv_do_drained_end(BlockDriverState *bs, BdrvChild *parent)
     assert(bs->quiesce_counter > 0);
 
     /* Re-enable things in child-to-parent order */
-    old_quiesce_counter = qatomic_fetch_dec(&bs->quiesce_counter);
-    if (old_quiesce_counter == 1) {
+    if (--bs->quiesce_counter == 0) {
         GRAPH_RDLOCK_GUARD_MAINLOOP();
         if (bs->drv && bs->drv->bdrv_drain_end) {
             bs->drv->bdrv_drain_end(bs);

include/block/block_int-common.h

@@ -1253,7 +1253,7 @@ struct BlockDriverState {
     /* do we need to tell the quest if we have a volatile write cache? */
     int enable_write_cache;
 
-    /* Accessed with atomic ops.  */
+    /* Accessed only in the main thread. */
     int quiesce_counter;
 
     unsigned int write_gen;               /* Current data generation */