From ac2ae233a0c266676195c0374195ec57197076cf Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Mon, 11 Sep 2023 11:46:04 +0200 Subject: [PATCH] block: Introduce bdrv_schedule_unref() bdrv_unref() is called by a lot of places that need to hold the graph lock (it naturally happens in the context of operations that change the graph). However, bdrv_unref() takes the graph writer lock internally, so it can't actually be called while already holding a graph lock without causing a deadlock. bdrv_unref() also can't just become GRAPH_WRLOCK because it drains the node before closing it, and draining requires that the graph is unlocked. The solution is to defer deleting the node until we don't hold the lock any more and draining is possible again. Note that keeping images open for longer than necessary can create problems, too: You can't open an image again before it is really closed (if image locking didn't prevent it, it would cause corruption). Reopening an image immediately happens at least during bdrv_open() and bdrv_co_create(). In order to solve this problem, make sure to run the deferred unref in bdrv_graph_wrunlock(), i.e. the first possible place where we can drain again. This is also why bdrv_schedule_unref() is marked GRAPH_WRLOCK. The output of iotest 051 is updated because the additional polling changes the order of HMP output, resulting in a new "(qemu)" prompt in the test output that was previously on a separate line and filtered out. Signed-off-by: Kevin Wolf Message-ID: <20230911094620.45040-6-kwolf@redhat.com> Reviewed-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- block.c | 17 +++++++++++++++++ block/graph-lock.c | 28 ++++++++++++++++++++-------- include/block/block-global-state.h | 1 + tests/qemu-iotests/051.pc.out | 6 +++--- 4 files changed, 41 insertions(+), 11 deletions(-) diff --git a/block.c b/block.c index 9029ddd9ff..c8ac7cfac4 100644 --- a/block.c +++ b/block.c @@ -7044,6 +7044,23 @@ void bdrv_unref(BlockDriverState *bs) } } +/* + * Release a BlockDriverState reference while holding the graph write lock. + * + * Calling bdrv_unref() directly is forbidden while holding the graph lock + * because bdrv_close() both involves polling and taking the graph lock + * internally. bdrv_schedule_unref() instead delays decreasing the refcount and + * possibly closing @bs until the graph lock is released. + */ +void bdrv_schedule_unref(BlockDriverState *bs) +{ + if (!bs) { + return; + } + aio_bh_schedule_oneshot(qemu_get_aio_context(), + (QEMUBHFunc *) bdrv_unref, bs); +} + struct BdrvOpBlocker { Error *reason; QLIST_ENTRY(BdrvOpBlocker) list; diff --git a/block/graph-lock.c b/block/graph-lock.c index f357a2c0b1..58a799065f 100644 --- a/block/graph-lock.c +++ b/block/graph-lock.c @@ -163,17 +163,29 @@ void bdrv_graph_wrlock(BlockDriverState *bs) void bdrv_graph_wrunlock(void) { GLOBAL_STATE_CODE(); - QEMU_LOCK_GUARD(&aio_context_list_lock); assert(qatomic_read(&has_writer)); - /* - * No need for memory barriers, this works in pair with - * the slow path of rdlock() and both take the lock. - */ - qatomic_store_release(&has_writer, 0); + WITH_QEMU_LOCK_GUARD(&aio_context_list_lock) { + /* + * No need for memory barriers, this works in pair with + * the slow path of rdlock() and both take the lock. + */ + qatomic_store_release(&has_writer, 0); - /* Wake up all coroutine that are waiting to read the graph */ - qemu_co_enter_all(&reader_queue, &aio_context_list_lock); + /* Wake up all coroutines that are waiting to read the graph */ + qemu_co_enter_all(&reader_queue, &aio_context_list_lock); + } + + /* + * Run any BHs that were scheduled during the wrlock section and that + * callers might expect to have finished (in particular, this is important + * for bdrv_schedule_unref()). + * + * Do this only after restarting coroutines so that nested event loops in + * BHs don't deadlock if their condition relies on the coroutine making + * progress. + */ + aio_bh_poll(qemu_get_aio_context()); } void coroutine_fn bdrv_graph_co_rdlock(void) diff --git a/include/block/block-global-state.h b/include/block/block-global-state.h index f347199bff..e570799f85 100644 --- a/include/block/block-global-state.h +++ b/include/block/block-global-state.h @@ -224,6 +224,7 @@ void bdrv_img_create(const char *filename, const char *fmt, void bdrv_ref(BlockDriverState *bs); void no_coroutine_fn bdrv_unref(BlockDriverState *bs); void coroutine_fn no_co_wrapper bdrv_co_unref(BlockDriverState *bs); +void GRAPH_WRLOCK bdrv_schedule_unref(BlockDriverState *bs); void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child); BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs, BlockDriverState *child_bs, diff --git a/tests/qemu-iotests/051.pc.out b/tests/qemu-iotests/051.pc.out index 4d4af5a486..7e10c5fa1b 100644 --- a/tests/qemu-iotests/051.pc.out +++ b/tests/qemu-iotests/051.pc.out @@ -169,11 +169,11 @@ QEMU_PROG: -device scsi-hd,drive=disk: Device needs media, but drive is empty Testing: -drive file=TEST_DIR/t.qcow2,if=none,node-name=disk -object iothread,id=thread0 -device virtio-scsi,iothread=thread0,id=virtio-scsi0 -device scsi-hd,bus=virtio-scsi0.0,drive=disk,share-rw=on -device ide-hd,drive=disk,share-rw=on QEMU X.Y.Z monitor - type 'help' for more information -QEMU_PROG: -device ide-hd,drive=disk,share-rw=on: Cannot change iothread of active block backend +(qemu) QEMU_PROG: -device ide-hd,drive=disk,share-rw=on: Cannot change iothread of active block backend Testing: -drive file=TEST_DIR/t.qcow2,if=none,node-name=disk -object iothread,id=thread0 -device virtio-scsi,iothread=thread0,id=virtio-scsi0 -device scsi-hd,bus=virtio-scsi0.0,drive=disk,share-rw=on -device virtio-blk-pci,drive=disk,share-rw=on QEMU X.Y.Z monitor - type 'help' for more information -QEMU_PROG: -device virtio-blk-pci,drive=disk,share-rw=on: Cannot change iothread of active block backend +(qemu) QEMU_PROG: -device virtio-blk-pci,drive=disk,share-rw=on: Cannot change iothread of active block backend Testing: -drive file=TEST_DIR/t.qcow2,if=none,node-name=disk -object iothread,id=thread0 -device virtio-scsi,iothread=thread0,id=virtio-scsi0 -device scsi-hd,bus=virtio-scsi0.0,drive=disk,share-rw=on -device lsi53c895a,id=lsi0 -device scsi-hd,bus=lsi0.0,drive=disk,share-rw=on QEMU X.Y.Z monitor - type 'help' for more information @@ -185,7 +185,7 @@ QEMU X.Y.Z monitor - type 'help' for more information Testing: -drive file=TEST_DIR/t.qcow2,if=none,node-name=disk -object iothread,id=thread0 -device virtio-scsi,iothread=thread0,id=virtio-scsi0 -device scsi-hd,bus=virtio-scsi0.0,drive=disk,share-rw=on -device virtio-blk-pci,drive=disk,iothread=thread0,share-rw=on QEMU X.Y.Z monitor - type 'help' for more information -QEMU_PROG: -device virtio-blk-pci,drive=disk,iothread=thread0,share-rw=on: Cannot change iothread of active block backend +(qemu) QEMU_PROG: -device virtio-blk-pci,drive=disk,iothread=thread0,share-rw=on: Cannot change iothread of active block backend Testing: -drive file=TEST_DIR/t.qcow2,if=none,node-name=disk -object iothread,id=thread0 -device virtio-scsi,iothread=thread0,id=virtio-scsi0 -device scsi-hd,bus=virtio-scsi0.0,drive=disk,share-rw=on -device virtio-scsi,id=virtio-scsi1,iothread=thread0 -device scsi-hd,bus=virtio-scsi1.0,drive=disk,share-rw=on QEMU X.Y.Z monitor - type 'help' for more information