From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Fiona Ebner <f.ebner@proxmox.com>
Date: Thu, 28 Sep 2023 10:07:03 +0200
Subject: [PATCH] Revert "Revert "graph-lock: Disable locking for now""

This reverts commit 3cce22defb4b0e47cf135444e30cc673cff5ebad.

There are still some issues with graph locking, e.g. deadlocks during
backup canceling [0]. Because the AioContext locks still exist, it
should be safe to disable locking again.

From the original 80fc5d2600 ("graph-lock: Disable locking for now"):

> We don't currently rely on graph locking yet. It is supposed to replace
> the AioContext lock eventually to enable multiqueue support, but as long
> as we still have the AioContext lock, it is sufficient without the graph
> lock. Once the AioContext lock goes away, the deadlock doesn't exist any
> more either and this commit can be reverted. (Of course, it can also be
> reverted while the AioContext lock still exists if the callers have been
> fixed.)

[0]: https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg00729.html

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
 block/graph-lock.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/block/graph-lock.c b/block/graph-lock.c
index 5e66f01ae8..5c2873262a 100644
--- a/block/graph-lock.c
+++ b/block/graph-lock.c
@@ -30,8 +30,10 @@ 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.
@@ -88,6 +90,7 @@ void unregister_aiocontext(AioContext *ctx)
     g_free(ctx->bdrv_graph);
 }
 
+#if 0
 static uint32_t reader_count(void)
 {
     BdrvGraphRWlock *brdv_graph;
@@ -105,12 +108,19 @@ static uint32_t reader_count(void)
     assert((int32_t)rd >= 0);
     return rd;
 }
+#endif
 
 void bdrv_graph_wrlock(BlockDriverState *bs)
 {
+#if 0
     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).
+     */
     assert(!qatomic_read(&has_writer));
 
     /*
@@ -158,11 +168,13 @@ void bdrv_graph_wrlock(BlockDriverState *bs)
     if (ctx) {
         aio_context_acquire(bdrv_get_aio_context(bs));
     }
+#endif
 }
 
 void bdrv_graph_wrunlock(void)
 {
     GLOBAL_STATE_CODE();
+#if 0
     QEMU_LOCK_GUARD(&aio_context_list_lock);
     assert(qatomic_read(&has_writer));
 
@@ -174,10 +186,13 @@ 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;
 
@@ -237,10 +252,12 @@ 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;
 
@@ -258,6 +275,7 @@ void coroutine_fn bdrv_graph_co_rdunlock(void)
     if (qatomic_read(&has_writer)) {
         aio_wait_kick();
     }
+#endif
 }
 
 void bdrv_graph_rdlock_main_loop(void)
@@ -275,13 +293,19 @@ 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
 }