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block: move drain outside of bdrv_try_change_aio_context()

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This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

Convert the function to a _locked() version that has to be called with
the graph lock held and add a convenience wrapper that has to be
called with the graph unlocked, which drains and takes the lock
itself. Since bdrv_try_change_aio_context() is global state code, the
wrapper is too.

Callers are adapted to use the appropriate variant, depending on
whether the caller already holds the lock. In the
test_set_aio_context() unit test, prior drains can be removed, because
draining already happens inside the new wrapper.

Note that bdrv_attach_child_common_abort(), bdrv_attach_child_common()
and bdrv_root_unref_child() hold the graph lock and are not actually
allowed to drain either. This will be addressed in the following
commits.

Functions like qmp_blockdev_mirror() query the nodes to act on before
draining and locking. In theory, draining could invalidate those nodes.
This kind of issue is not addressed by these commits.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-10-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Fiona Ebner 2025-05-30 17:10:46 +02:00 • committed by Kevin Wolf
parent 91ba0e1c38
commit a1ea8eb591
4 changed files with 59 additions and 26 deletions
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58
block.c
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@ -3028,7 +3028,10 @@ static void GRAPH_WRLOCK bdrv_attach_child_common_abort(void *opaque)
bdrv_replace_child_noperm(s->child, NULL);
if (bdrv_get_aio_context(bs) != s->old_child_ctx) {
bdrv_try_change_aio_context(bs, s->old_child_ctx, NULL, &error_abort);
bdrv_drain_all_begin();
bdrv_try_change_aio_context_locked(bs, s->old_child_ctx, NULL,
&error_abort);
bdrv_drain_all_end();
}
if (bdrv_child_get_parent_aio_context(s->child) != s->old_parent_ctx) {
@ -3115,8 +3118,10 @@ bdrv_attach_child_common(BlockDriverState *child_bs,
parent_ctx = bdrv_child_get_parent_aio_context(new_child);
if (child_ctx != parent_ctx) {
Error *local_err = NULL;
int ret = bdrv_try_change_aio_context(child_bs, parent_ctx, NULL,
&local_err);
bdrv_drain_all_begin();
int ret = bdrv_try_change_aio_context_locked(child_bs, parent_ctx, NULL,
&local_err);
bdrv_drain_all_end();
if (ret < 0 && child_class->change_aio_ctx) {
Transaction *aio_ctx_tran = tran_new();
@ -3319,8 +3324,10 @@ void bdrv_root_unref_child(BdrvChild *child)
* When the parent requiring a non-default AioContext is removed, the
* node moves back to the main AioContext
*/
bdrv_try_change_aio_context(child_bs, qemu_get_aio_context(), NULL,
NULL);
bdrv_drain_all_begin();
bdrv_try_change_aio_context_locked(child_bs, qemu_get_aio_context(),
NULL, NULL);
bdrv_drain_all_end();
}
bdrv_schedule_unref(child_bs);
@ -7719,9 +7726,13 @@ bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
*
* If ignore_child is not NULL, that child (and its subgraph) will not
* be touched.
*
* Called with the graph lock held.
*
* Called while all bs are drained.
*/
int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
BdrvChild *ignore_child, Error **errp)
int bdrv_try_change_aio_context_locked(BlockDriverState *bs, AioContext *ctx,
BdrvChild *ignore_child, Error **errp)
{
Transaction *tran;
GHashTable *visited;
@ -7730,17 +7741,15 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
/*
* Recursion phase: go through all nodes of the graph.
* Take care of checking that all nodes support changing AioContext
* and drain them, building a linear list of callbacks to run if everything
* is successful (the transaction itself).
* Take care of checking that all nodes support changing AioContext,
* building a linear list of callbacks to run if everything is successful
* (the transaction itself).
*/
tran = tran_new();
visited = g_hash_table_new(NULL, NULL);
if (ignore_child) {
g_hash_table_add(visited, ignore_child);
}
bdrv_drain_all_begin();
bdrv_graph_rdlock_main_loop();
ret = bdrv_change_aio_context(bs, ctx, visited, tran, errp);
g_hash_table_destroy(visited);
@ -7754,15 +7763,34 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
if (!ret) {
/* Just run clean() callbacks. No AioContext changed. */
tran_abort(tran);
bdrv_graph_rdunlock_main_loop();
bdrv_drain_all_end();
return -EPERM;
}
tran_commit(tran);
return 0;
}
/*
* Change bs's and recursively all of its parents' and children's AioContext
* to the given new context, returning an error if that isn't possible.
*
* If ignore_child is not NULL, that child (and its subgraph) will not
* be touched.
*/
int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
BdrvChild *ignore_child, Error **errp)
{
int ret;
GLOBAL_STATE_CODE();
bdrv_drain_all_begin();
bdrv_graph_rdlock_main_loop();
ret = bdrv_try_change_aio_context_locked(bs, ctx, ignore_child, errp);
bdrv_graph_rdunlock_main_loop();
bdrv_drain_all_end();
return 0;
return ret;
}
void bdrv_add_aio_context_notifier(BlockDriverState *bs,