aio-wait: avoid AioContext lock in aio_wait_bh_oneshot()

There is no need for the AioContext lock in aio_wait_bh_oneshot().
It's easy to remove the lock from existing callers and then switch from
AIO_WAIT_WHILE() to AIO_WAIT_WHILE_UNLOCKED() in aio_wait_bh_oneshot().

Document that the AioContext lock should not be held across
aio_wait_bh_oneshot(). Holding a lock across aio_poll() can cause
deadlock so we don't want callers to do that.

This is a step towards getting rid of the AioContext lock.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230404153307.458883-1-stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index a6202997eeefd922df3b00de4da11bc8eb932f9c..af1c24c40c1f2ca61b3f29bf3067222d5d42f05e 100644 (file)
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -315,9 +315,10 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
     s->stopping = true;
     trace_virtio_blk_data_plane_stop(s);
 
-    aio_context_acquire(s->ctx);
     aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
 
+    aio_context_acquire(s->ctx);
+
     /* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */
     blk_drain(s->conf->conf.blk);
 

diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index 20bb91766eede2c1bea218bfc40908d4bd0f31e7..f3214e1c57e52a5c535641acc4e73e263216b5fd 100644 (file)
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -197,9 +197,7 @@ void virtio_scsi_dataplane_stop(VirtIODevice *vdev)
     }
     s->dataplane_stopping = true;
 
-    aio_context_acquire(s->ctx);
     aio_wait_bh_oneshot(s->ctx, virtio_scsi_dataplane_stop_bh, s);
-    aio_context_release(s->ctx);
 
     blk_drain_all(); /* ensure there are no in-flight requests */
 

diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
index 6e43e3b7bb28e86a50a4c2220b2a93c8b8f6965b..5449b6d7428df3e9604c880b630c6f0565bb1dc2 100644 (file)
--- a/include/block/aio-wait.h
+++ b/include/block/aio-wait.h
@@ -131,7 +131,7 @@ void aio_wait_kick(void);
  *
  * Run a BH in @ctx and wait for it to complete.
  *
- * Must be called from the main loop thread with @ctx acquired exactly once.
+ * Must be called from the main loop thread without @ctx acquired.
  * Note that main loop event processing may occur.
  */
 void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);

diff --git a/util/aio-wait.c b/util/aio-wait.c
index 98c5accd29d0f2999b59e3a898e74ba9a46ce812..b5336cf5fd2932dd0f54d98c96c68a4a16231bd7 100644 (file)
--- a/util/aio-wait.c
+++ b/util/aio-wait.c
@@ -82,5 +82,5 @@ void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
     assert(qemu_get_current_aio_context() == qemu_get_aio_context());
 
     aio_bh_schedule_oneshot(ctx, aio_wait_bh, &data);
-    AIO_WAIT_WHILE(ctx, !data.done);
+    AIO_WAIT_WHILE_UNLOCKED(NULL, !data.done);
 }