[2/8] block-backend: enable_write_cache should be atomic

Commit Message

Emanuele Giuseppe Esposito June 9, 2022, 2:37 p.m. UTC

It is read from IO_CODE and written with BQL held,
so setting it as atomic should be enough.

Also remove the aiocontext lock that was sporadically
taken around the set.

Signed-off-by: Emanuele Giuseppe Esposito <[e-mail]>
---
 block/block-backend.c | 6 +++---
 hw/block/virtio-blk.c | 4 ----
 2 files changed, 3 insertions(+), 7 deletions(-)

Comments

Stefan Hajnoczi July 5, 2022, 2:16 p.m. UTC | #1

On Thu, Jun 09, 2022 at 10:37:21AM -0400, Emanuele Giuseppe Esposito wrote:
> It is read from IO_CODE and written with BQL held,
> so setting it as atomic should be enough.
> 
> Also remove the aiocontext lock that was sporadically
> taken around the set.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <[e-mail]>
> ---
>  block/block-backend.c | 6 +++---
>  hw/block/virtio-blk.c | 4 ----
>  2 files changed, 3 insertions(+), 7 deletions(-)

Reviewed-by: Stefan Hajnoczi <[e-mail]>

Patch

diff --git a/block/block-backend.c b/block/block-backend.c
index f425b00793..384e52d564 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -60,7 +60,7 @@  struct BlockBackend {
      * can be used to restore those options in the new BDS on insert) */
     BlockBackendRootState root_state;
 
-    bool enable_write_cache;
+    bool enable_write_cache; /* Atomic */
 
     /* I/O stats (display with "info blockstats"). */
     BlockAcctStats stats;
@@ -1972,13 +1972,13 @@  bool blk_is_sg(BlockBackend *blk)
 bool blk_enable_write_cache(BlockBackend *blk)
 {
     IO_CODE();
-    return blk->enable_write_cache;
+    return qatomic_read(&blk->enable_write_cache);
 }
 
 void blk_set_enable_write_cache(BlockBackend *blk, bool wce)
 {
     GLOBAL_STATE_CODE();
-    blk->enable_write_cache = wce;
+    qatomic_set(&blk->enable_write_cache, wce);
 }
 
 void blk_activate(BlockBackend *blk, Error **errp)
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 8d0590cc76..191f75ce25 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -988,9 +988,7 @@  static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
 
     memcpy(&blkcfg, config, s->config_size);
 
-    aio_context_acquire(blk_get_aio_context(s->blk));
     blk_set_enable_write_cache(s->blk, blkcfg.wce != 0);
-    aio_context_release(blk_get_aio_context(s->blk));
 }
 
 static uint64_t virtio_blk_get_features(VirtIODevice *vdev, uint64_t features,
@@ -1058,11 +1056,9 @@  static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t status)
      * s->blk would erroneously be placed in writethrough mode.
      */
     if (!virtio_vdev_has_feature(vdev, VIRTIO_BLK_F_CONFIG_WCE)) {
-        aio_context_acquire(blk_get_aio_context(s->blk));
         blk_set_enable_write_cache(s->blk,
                                    virtio_vdev_has_feature(vdev,
                                                            VIRTIO_BLK_F_WCE));
-        aio_context_release(blk_get_aio_context(s->blk));
     }
 }