block: remove outdated AioContext locking comments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 <stefanha@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-ID: <20231205182011.1976568-15-stefanha@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2023-12-05 13:20:11 -05:00 · 2023-12-05 13:20:11 -05:00 · 23c983c8f6
commit 23c983c8f6
parent e91083cd3f
8 changed files with 22 additions and 82 deletions
--- 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).
     */

--- 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)
 {
--- 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)
 {
--- 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
--- 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.
--- 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
--- 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

--- 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