From 23c983c8f6b2fea22698f501aa6a2c03dadd81ba Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 5 Dec 2023 13:20:11 -0500 Subject: [PATCH] block: remove outdated AioContext locking comments The AioContext lock no longer exists. There is one noteworthy change: - * More specifically, these functions use BDRV_POLL_WHILE(bs), which - * requires the caller to be either in the main thread and hold - * the BlockdriverState (bs) AioContext lock, or directly in the - * home thread that runs the bs AioContext. Calling them from - * another thread in another AioContext would cause deadlocks. + * More specifically, these functions use BDRV_POLL_WHILE(bs), which requires + * the caller to be either in the main thread or directly in the home thread + * that runs the bs AioContext. Calling them from another thread in another + * AioContext would cause deadlocks. I am not sure whether deadlocks are still possible. Maybe they have just moved to the fine-grained locks that have replaced the AioContext. Since I am not sure if the deadlocks are gone, I have kept the substance unchanged and just removed mention of the AioContext. Signed-off-by: Stefan Hajnoczi Reviewed-by: Eric Blake Message-ID: <20231205182011.1976568-15-stefanha@redhat.com> Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- block.c | 73 ++++++---------------------- block/block-backend.c | 8 --- block/export/vhost-user-blk-server.c | 4 -- include/block/block-common.h | 3 -- include/block/block-io.h | 9 ++-- include/block/block_int-common.h | 2 - tests/qemu-iotests/202 | 2 +- tests/qemu-iotests/203 | 3 +- 8 files changed, 22 insertions(+), 82 deletions(-) diff --git a/block.c b/block.c index 434b7f4d72..a097772238 100644 --- a/block.c +++ b/block.c @@ -1616,11 +1616,6 @@ out: g_free(gen_node_name); } -/* - * The caller must always hold @bs AioContext lock, because this function calls - * bdrv_refresh_total_sectors() which polls when called from non-coroutine - * context. - */ static int no_coroutine_fn GRAPH_UNLOCKED bdrv_open_driver(BlockDriverState *bs, BlockDriver *drv, const char *node_name, QDict *options, int open_flags, Error **errp) @@ -2901,7 +2896,7 @@ uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm) * Replaces the node that a BdrvChild points to without updating permissions. * * If @new_bs is non-NULL, the parent of @child must already be drained through - * @child and the caller must hold the AioContext lock for @new_bs. + * @child. */ static void GRAPH_WRLOCK bdrv_replace_child_noperm(BdrvChild *child, BlockDriverState *new_bs) @@ -3041,9 +3036,8 @@ static TransactionActionDrv bdrv_attach_child_common_drv = { * * Returns new created child. * - * The caller must hold the AioContext lock for @child_bs. Both @parent_bs and - * @child_bs can move to a different AioContext in this function. Callers must - * make sure that their AioContext locking is still correct after this. + * Both @parent_bs and @child_bs can move to a different AioContext in this + * function. */ static BdrvChild * GRAPH_WRLOCK bdrv_attach_child_common(BlockDriverState *child_bs, @@ -3142,9 +3136,8 @@ bdrv_attach_child_common(BlockDriverState *child_bs, /* * Function doesn't update permissions, caller is responsible for this. * - * The caller must hold the AioContext lock for @child_bs. Both @parent_bs and - * @child_bs can move to a different AioContext in this function. Callers must - * make sure that their AioContext locking is still correct after this. + * Both @parent_bs and @child_bs can move to a different AioContext in this + * function. * * After calling this function, the transaction @tran may only be completed * while holding a writer lock for the graph. @@ -3184,9 +3177,6 @@ bdrv_attach_child_noperm(BlockDriverState *parent_bs, * * On failure NULL is returned, errp is set and the reference to * child_bs is also dropped. - * - * The caller must hold the AioContext lock @child_bs, but not that of @ctx - * (unless @child_bs is already in @ctx). */ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs, const char *child_name, @@ -3226,9 +3216,6 @@ out: * * On failure NULL is returned, errp is set and the reference to * child_bs is also dropped. - * - * If @parent_bs and @child_bs are in different AioContexts, the caller must - * hold the AioContext lock for @child_bs, but not for @parent_bs. */ BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs, BlockDriverState *child_bs, @@ -3418,9 +3405,8 @@ static BdrvChildRole bdrv_backing_role(BlockDriverState *bs) * * Function doesn't update permissions, caller is responsible for this. * - * The caller must hold the AioContext lock for @child_bs. Both @parent_bs and - * @child_bs can move to a different AioContext in this function. Callers must - * make sure that their AioContext locking is still correct after this. + * Both @parent_bs and @child_bs can move to a different AioContext in this + * function. * * After calling this function, the transaction @tran may only be completed * while holding a writer lock for the graph. @@ -3513,9 +3499,8 @@ out: } /* - * The caller must hold the AioContext lock for @backing_hd. Both @bs and - * @backing_hd can move to a different AioContext in this function. Callers must - * make sure that their AioContext locking is still correct after this. + * Both @bs and @backing_hd can move to a different AioContext in this + * function. * * If a backing child is already present (i.e. we're detaching a node), that * child node must be drained. @@ -3574,8 +3559,6 @@ int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd, * itself, all options starting with "${bdref_key}." are considered part of the * BlockdevRef. * - * The caller must hold the main AioContext lock. - * * TODO Can this be unified with bdrv_open_image()? */ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options, @@ -3745,9 +3728,7 @@ done: * * The BlockdevRef will be removed from the options QDict. * - * The caller must hold the lock of the main AioContext and no other AioContext. - * @parent can move to a different AioContext in this function. Callers must - * make sure that their AioContext locking is still correct after this. + * @parent can move to a different AioContext in this function. */ BdrvChild *bdrv_open_child(const char *filename, QDict *options, const char *bdref_key, @@ -3778,9 +3759,7 @@ BdrvChild *bdrv_open_child(const char *filename, /* * Wrapper on bdrv_open_child() for most popular case: open primary child of bs. * - * The caller must hold the lock of the main AioContext and no other AioContext. - * @parent can move to a different AioContext in this function. Callers must - * make sure that their AioContext locking is still correct after this. + * @parent can move to a different AioContext in this function. */ int bdrv_open_file_child(const char *filename, QDict *options, const char *bdref_key, @@ -3923,8 +3902,6 @@ out: * The reference parameter may be used to specify an existing block device which * should be opened. If specified, neither options nor a filename may be given, * nor can an existing BDS be reused (that is, *pbs has to be NULL). - * - * The caller must always hold the main AioContext lock. */ static BlockDriverState * no_coroutine_fn bdrv_open_inherit(const char *filename, const char *reference, QDict *options, @@ -4217,7 +4194,6 @@ close_and_fail: return NULL; } -/* The caller must always hold the main AioContext lock. */ BlockDriverState *bdrv_open(const char *filename, const char *reference, QDict *options, int flags, Error **errp) { @@ -4665,10 +4641,7 @@ int bdrv_reopen_set_read_only(BlockDriverState *bs, bool read_only, * * Return 0 on success, otherwise return < 0 and set @errp. * - * The caller must hold the AioContext lock of @reopen_state->bs. * @reopen_state->bs can move to a different AioContext in this function. - * Callers must make sure that their AioContext locking is still correct after - * this. */ static int GRAPH_UNLOCKED bdrv_reopen_parse_file_or_backing(BDRVReopenState *reopen_state, @@ -4801,8 +4774,6 @@ out_rdlock: * It is the responsibility of the caller to then call the abort() or * commit() for any other BDS that have been left in a prepare() state * - * The caller must hold the AioContext lock of @reopen_state->bs. - * * After calling this function, the transaction @change_child_tran may only be * completed while holding a writer lock for the graph. */ @@ -5437,8 +5408,6 @@ int bdrv_drop_filter(BlockDriverState *bs, Error **errp) * child. * * This function does not create any image files. - * - * The caller must hold the AioContext lock for @bs_top. */ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top, Error **errp) @@ -5545,9 +5514,8 @@ static void bdrv_delete(BlockDriverState *bs) * after the call (even on failure), so if the caller intends to reuse the * dictionary, it needs to use qobject_ref() before calling bdrv_open. * - * The caller holds the AioContext lock for @bs. It must make sure that @bs - * stays in the same AioContext, i.e. @options must not refer to nodes in a - * different AioContext. + * The caller must make sure that @bs stays in the same AioContext, i.e. + * @options must not refer to nodes in a different AioContext. */ BlockDriverState *bdrv_insert_node(BlockDriverState *bs, QDict *options, int flags, Error **errp) @@ -7565,10 +7533,6 @@ static TransactionActionDrv set_aio_context = { * * Must be called from the main AioContext. * - * The caller must own the AioContext lock for the old AioContext of bs, but it - * must not own the AioContext lock for new_context (unless new_context is the - * same as the current context of bs). - * * @visited will accumulate all visited BdrvChild objects. The caller is * responsible for freeing the list afterwards. */ @@ -7621,13 +7585,6 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, * * If ignore_child is not NULL, that child (and its subgraph) will not * be touched. - * - * This function still requires the caller to take the bs current - * AioContext lock, otherwise draining will fail since AIO_WAIT_WHILE - * assumes the lock is always held if bs is in another AioContext. - * For the same reason, it temporarily also holds the new AioContext, since - * bdrv_drained_end calls BDRV_POLL_WHILE that assumes the lock is taken too. - * Therefore the new AioContext lock must not be taken by the caller. */ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx, BdrvChild *ignore_child, Error **errp) @@ -7653,8 +7610,8 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx, /* * Linear phase: go through all callbacks collected in the transaction. - * Run all callbacks collected in the recursion to switch all nodes - * AioContext lock (transaction commit), or undo all changes done in the + * Run all callbacks collected in the recursion to switch every node's + * AioContext (transaction commit), or undo all changes done in the * recursion (transaction abort). */ diff --git a/block/block-backend.c b/block/block-backend.c index f412bed274..209eb07528 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -390,8 +390,6 @@ BlockBackend *blk_new(AioContext *ctx, uint64_t perm, uint64_t shared_perm) * Both sets of permissions can be changed later using blk_set_perm(). * * Return the new BlockBackend on success, null on failure. - * - * Callers must hold the AioContext lock of @bs. */ BlockBackend *blk_new_with_bs(BlockDriverState *bs, uint64_t perm, uint64_t shared_perm, Error **errp) @@ -416,8 +414,6 @@ BlockBackend *blk_new_with_bs(BlockDriverState *bs, uint64_t perm, * Just as with bdrv_open(), after having called this function the reference to * @options belongs to the block layer (even on failure). * - * Called without holding an AioContext lock. - * * TODO: Remove @filename and @flags; it should be possible to specify a whole * BDS tree just by specifying the @options QDict (or @reference, * alternatively). At the time of adding this function, this is not possible, @@ -872,8 +868,6 @@ BlockBackend *blk_by_public(BlockBackendPublic *public) /* * Disassociates the currently associated BlockDriverState from @blk. - * - * The caller must hold the AioContext lock for the BlockBackend. */ void blk_remove_bs(BlockBackend *blk) { @@ -915,8 +909,6 @@ void blk_remove_bs(BlockBackend *blk) /* * Associates a new BlockDriverState with @blk. - * - * Callers must hold the AioContext lock of @bs. */ int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp) { diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c index 16f48388d3..50c358e8cd 100644 --- a/block/export/vhost-user-blk-server.c +++ b/block/export/vhost-user-blk-server.c @@ -278,7 +278,6 @@ static void vu_blk_exp_resize(void *opaque) vu_config_change_msg(&vexp->vu_server.vu_dev); } -/* Called with vexp->export.ctx acquired */ static void vu_blk_drained_begin(void *opaque) { VuBlkExport *vexp = opaque; @@ -287,7 +286,6 @@ static void vu_blk_drained_begin(void *opaque) vhost_user_server_detach_aio_context(&vexp->vu_server); } -/* Called with vexp->export.blk AioContext acquired */ static void vu_blk_drained_end(void *opaque) { VuBlkExport *vexp = opaque; @@ -300,8 +298,6 @@ static void vu_blk_drained_end(void *opaque) * Ensures that bdrv_drained_begin() waits until in-flight requests complete * and the server->co_trip coroutine has terminated. It will be restarted in * vhost_user_server_attach_aio_context(). - * - * Called with vexp->export.ctx acquired. */ static bool vu_blk_drained_poll(void *opaque) { diff --git a/include/block/block-common.h b/include/block/block-common.h index d7599564db..a846023a09 100644 --- a/include/block/block-common.h +++ b/include/block/block-common.h @@ -70,9 +70,6 @@ * automatically takes the graph rdlock when calling the wrapped function. In * the same way, no_co_wrapper_bdrv_wrlock functions automatically take the * graph wrlock. - * - * If the first parameter of the function is a BlockDriverState, BdrvChild or - * BlockBackend pointer, the AioContext lock for it is taken in the wrapper. */ #define no_co_wrapper #define no_co_wrapper_bdrv_rdlock diff --git a/include/block/block-io.h b/include/block/block-io.h index 8eb39a858b..b49e0537dd 100644 --- a/include/block/block-io.h +++ b/include/block/block-io.h @@ -332,11 +332,10 @@ bdrv_co_copy_range(BdrvChild *src, int64_t src_offset, * "I/O or GS" API functions. These functions can run without * the BQL, but only in one specific iothread/main loop. * - * More specifically, these functions use BDRV_POLL_WHILE(bs), which - * requires the caller to be either in the main thread and hold - * the BlockdriverState (bs) AioContext lock, or directly in the - * home thread that runs the bs AioContext. Calling them from - * another thread in another AioContext would cause deadlocks. + * More specifically, these functions use BDRV_POLL_WHILE(bs), which requires + * the caller to be either in the main thread or directly in the home thread + * that runs the bs AioContext. Calling them from another thread in another + * AioContext would cause deadlocks. * * Therefore, these functions are not proper I/O, because they * can't run in *any* iothreads, but only in a specific one. diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h index 4e31d161c5..151279d481 100644 --- a/include/block/block_int-common.h +++ b/include/block/block_int-common.h @@ -1192,8 +1192,6 @@ struct BlockDriverState { /* The error object in use for blocking operations on backing_hd */ Error *backing_blocker; - /* Protected by AioContext lock */ - /* * If we are reading a disk image, give its size in sectors. * Generally read-only; it is written to by load_snapshot and diff --git a/tests/qemu-iotests/202 b/tests/qemu-iotests/202 index b784dcd791..13304242e5 100755 --- a/tests/qemu-iotests/202 +++ b/tests/qemu-iotests/202 @@ -21,7 +21,7 @@ # Check that QMP 'transaction' blockdev-snapshot-sync with multiple drives on a # single IOThread completes successfully. This particular command triggered a # hang due to recursive AioContext locking and BDRV_POLL_WHILE(). Protect -# against regressions. +# against regressions even though the AioContext lock no longer exists. import iotests diff --git a/tests/qemu-iotests/203 b/tests/qemu-iotests/203 index ab80fd0e44..1ba878522b 100755 --- a/tests/qemu-iotests/203 +++ b/tests/qemu-iotests/203 @@ -21,7 +21,8 @@ # Check that QMP 'migrate' with multiple drives on a single IOThread completes # successfully. This particular command triggered a hang in the source QEMU # process due to recursive AioContext locking in bdrv_invalidate_all() and -# BDRV_POLL_WHILE(). +# BDRV_POLL_WHILE(). Protect against regressions even though the AioContext +# lock no longer exists. import iotests