[PATCH v2 23/24] block: never use atomics to access bs->quiesce_counter

[Fiona Ebner]

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.eb...@proxmox.com>
---

New in v2.

 block/io.c                       | 7 ++-----
 include/block/block_int-common.h | 2 +-
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/block/io.c b/block/io.c
index ac5c7174f6..9bd8ba8431 100644
--- a/block/io.c
+++ b/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);
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index 37466c7841..f9c60c1ae0 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -1227,7 +1227,7 @@ 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 */
-- 
2.39.5