Revert "graph-lock: Disable locking for now"

Now that bdrv_graph_wrlock() temporarily drops the AioContext lock that
its caller holds, it can poll without causing deadlocks. We can now
re-enable graph locking.

This reverts commit ad128dff0bf4b6f971d05eb4335a627883a19c1d.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-12-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/graph-lock.c

@@ -30,10 +30,8 @@ BdrvGraphLock graph_lock;
 /* Protects the list of aiocontext and orphaned_reader_count */
 static QemuMutex aio_context_list_lock;
 
-#if 0
 /* Written and read with atomic operations. */
 static int has_writer;
-#endif
 
 /*
  * A reader coroutine could move from an AioContext to another.
@@ -90,7 +88,6 @@ void unregister_aiocontext(AioContext *ctx)
     g_free(ctx->bdrv_graph);
 }
 
-#if 0
 static uint32_t reader_count(void)
 {
     BdrvGraphRWlock *brdv_graph;
@@ -108,21 +105,13 @@ static uint32_t reader_count(void)
     assert((int32_t)rd >= 0);
     return rd;
 }
-#endif
 
 void bdrv_graph_wrlock(BlockDriverState *bs)
 {
     AioContext *ctx = NULL;
 
     GLOBAL_STATE_CODE();
-    /*
-     * TODO Some callers hold an AioContext lock when this is called, which
-     * causes deadlocks. Reenable once the AioContext locking is cleaned up (or
-     * AioContext locks are gone).
-     */
-#if 0
     assert(!qatomic_read(&has_writer));
-#endif
 
     /*
      * Release only non-mainloop AioContext. The mainloop often relies on the
@@ -137,7 +126,6 @@ void bdrv_graph_wrlock(BlockDriverState *bs)
         }
     }
 
-#if 0
     /* Make sure that constantly arriving new I/O doesn't cause starvation */
     bdrv_drain_all_begin_nopoll();
 
@@ -166,7 +154,6 @@ void bdrv_graph_wrlock(BlockDriverState *bs)
     } while (reader_count() >= 1);
 
     bdrv_drain_all_end();
-#endif
 
     if (ctx) {
         aio_context_acquire(bdrv_get_aio_context(bs));
@@ -176,7 +163,6 @@ void bdrv_graph_wrlock(BlockDriverState *bs)
 void bdrv_graph_wrunlock(void)
 {
     GLOBAL_STATE_CODE();
-#if 0
     QEMU_LOCK_GUARD(&aio_context_list_lock);
     assert(qatomic_read(&has_writer));
 
@@ -188,13 +174,10 @@ void bdrv_graph_wrunlock(void)
 
     /* Wake up all coroutine that are waiting to read the graph */
     qemu_co_enter_all(&reader_queue, &aio_context_list_lock);
-#endif
 }
 
 void coroutine_fn bdrv_graph_co_rdlock(void)
 {
-    /* TODO Reenable when wrlock is reenabled */
-#if 0
     BdrvGraphRWlock *bdrv_graph;
     bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
 
@@ -254,12 +237,10 @@ void coroutine_fn bdrv_graph_co_rdlock(void)
             qemu_co_queue_wait(&reader_queue, &aio_context_list_lock);
         }
     }
-#endif
 }
 
 void coroutine_fn bdrv_graph_co_rdunlock(void)
 {
-#if 0
     BdrvGraphRWlock *bdrv_graph;
     bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
 
@@ -277,7 +258,6 @@ void coroutine_fn bdrv_graph_co_rdunlock(void)
     if (qatomic_read(&has_writer)) {
         aio_wait_kick();
     }
-#endif
 }
 
 void bdrv_graph_rdlock_main_loop(void)
@@ -295,19 +275,13 @@ void bdrv_graph_rdunlock_main_loop(void)
 void assert_bdrv_graph_readable(void)
 {
     /* reader_count() is slow due to aio_context_list_lock lock contention */
-    /* TODO Reenable when wrlock is reenabled */
-#if 0
 #ifdef CONFIG_DEBUG_GRAPH_LOCK
     assert(qemu_in_main_thread() || reader_count());
 #endif
-#endif
 }
 
 void assert_bdrv_graph_writable(void)
 {
     assert(qemu_in_main_thread());
-    /* TODO Reenable when wrlock is reenabled */
-#if 0
     assert(qatomic_read(&has_writer));
-#endif
 }