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block: move drain outside of bdrv_change_aio_context() and mark GRAPH_RDLOCK

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This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED.

Note that even if bdrv_drained_begin() were already marked as
GRAPH_UNLOCKED, TSA would not complain about the instance in
bdrv_change_aio_context() before this change, because it is preceded
by a bdrv_graph_rdunlock_main_loop() call. It is not correct to
release the lock here, and in case the caller holds a write lock, it
wouldn't actually release the lock.

In combination with block-stream, there is a deadlock that can happen
because of this [0]. In particular, it can happen that
main thread              IO thread
1. acquires write lock
                         in blk_co_do_preadv_part():
                         2. have non-zero blk->in_flight
                         3. try to acquire read lock
4. begin drain

Steps 3 and 4 might be switched. Draining will poll and get stuck,
because it will see the non-zero in_flight counter. But the IO thread
will not make any progress either, because it cannot acquire the read
lock.

After this change, all paths to bdrv_change_aio_context() drain:
bdrv_change_aio_context() is called by:
1. bdrv_child_cb_change_aio_ctx() which is only called via the
   change_aio_ctx() callback, see below.
2. bdrv_child_change_aio_context(), see below.
3. bdrv_try_change_aio_context(), where a drained section is
   introduced.

The change_aio_ctx() callback is called by:
1. bdrv_attach_child_common_abort(), where a drained section is
   introduced.
2. bdrv_attach_child_common(), where a drained section is introduced.
3. bdrv_parent_change_aio_context(), see below.

bdrv_child_change_aio_context() is called by:
1. bdrv_change_aio_context(), i.e. recursive, so being in a drained
   section is invariant.
2. child_job_change_aio_ctx(), which is only called via the
   change_aio_ctx() callback, see above.

bdrv_parent_change_aio_context() is called by:
1. bdrv_change_aio_context(), i.e. recursive, so being in a drained
   section is invariant.

This resolves all code paths. Note that bdrv_attach_child_common()
and bdrv_attach_child_common_abort() hold the graph write lock and
callers of bdrv_try_change_aio_context() might too, so they are not
actually allowed to drain either. This will be addressed in the
following commits.

More granular draining is not trivially possible, because
bdrv_change_aio_context() can recursively call itself e.g. via
bdrv_child_change_aio_context().

[0]: https://lore.kernel.org/qemu-devel/73839c04-7616-407e-b057-80ca69e63f51@virtuozzo.com/

Reported-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-9-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Fiona Ebner 2025-05-30 17:10:45 +02:00 • committed by Kevin Wolf
parent 469422c45b
commit 91ba0e1c38
2 changed files with 53 additions and 16 deletions
Download patch file Download diff file Expand all files Collapse all files

57
block.c
View file

@ -106,9 +106,9 @@ static void bdrv_reopen_abort(BDRVReopenState *reopen_state);
static bool bdrv_backing_overridden(BlockDriverState *bs);
static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
GHashTable *visited, Transaction *tran,
Error **errp);
static bool GRAPH_RDLOCK
bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
GHashTable *visited, Transaction *tran, Error **errp);
/* If non-zero, use only whitelisted block drivers */
static int use_bdrv_whitelist;
@ -3040,8 +3040,10 @@ static void GRAPH_WRLOCK bdrv_attach_child_common_abort(void *opaque)
/* No need to visit `child`, because it has been detached already */
visited = g_hash_table_new(NULL, NULL);
bdrv_drain_all_begin();
ret = s->child->klass->change_aio_ctx(s->child, s->old_parent_ctx,
visited, tran, &error_abort);
bdrv_drain_all_end();
g_hash_table_destroy(visited);
/* transaction is supposed to always succeed */
@ -3122,9 +3124,11 @@ bdrv_attach_child_common(BlockDriverState *child_bs,
bool ret_child;
g_hash_table_add(visited, new_child);
bdrv_drain_all_begin();
ret_child = child_class->change_aio_ctx(new_child, child_ctx,
visited, aio_ctx_tran,
NULL);
bdrv_drain_all_end();
if (ret_child == true) {
error_free(local_err);
ret = 0;
@ -7576,6 +7580,17 @@ typedef struct BdrvStateSetAioContext {
BlockDriverState *bs;
} BdrvStateSetAioContext;
/*
* Changes the AioContext of @child to @ctx and recursively for the associated
* block nodes and all their children and parents. Returns true if the change is
* possible and the transaction @tran can be continued. Returns false and sets
* @errp if not and the transaction must be aborted.
*
* @visited will accumulate all visited BdrvChild objects. The caller is
* responsible for freeing the list afterwards.
*
* Must be called with the affected block nodes drained.
*/
static bool GRAPH_RDLOCK
bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx,
GHashTable *visited, Transaction *tran,
@ -7604,6 +7619,17 @@ bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx,
return true;
}
/*
* Changes the AioContext of @c->bs to @ctx and recursively for all its children
* and parents. Returns true if the change is possible and the transaction @tran
* can be continued. Returns false and sets @errp if not and the transaction
* must be aborted.
*
* @visited will accumulate all visited BdrvChild objects. The caller is
* responsible for freeing the list afterwards.
*
* Must be called with the affected block nodes drained.
*/
bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx,
GHashTable *visited, Transaction *tran,
Error **errp)
@ -7619,10 +7645,6 @@ bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx,
static void bdrv_set_aio_context_clean(void *opaque)
{
BdrvStateSetAioContext *state = (BdrvStateSetAioContext *) opaque;
BlockDriverState *bs = (BlockDriverState *) state->bs;
/* Paired with bdrv_drained_begin in bdrv_change_aio_context() */
bdrv_drained_end(bs);
g_free(state);
}
@ -7650,10 +7672,12 @@ static TransactionActionDrv set_aio_context = {
*
* @visited will accumulate all visited BdrvChild objects. The caller is
* responsible for freeing the list afterwards.
*
* @bs must be drained.
*/
static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
GHashTable *visited, Transaction *tran,
Error **errp)
static bool GRAPH_RDLOCK
bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
GHashTable *visited, Transaction *tran, Error **errp)
{
BdrvChild *c;
BdrvStateSetAioContext *state;
@ -7664,21 +7688,17 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
return true;
}
bdrv_graph_rdlock_main_loop();
QLIST_FOREACH(c, &bs->parents, next_parent) {
if (!bdrv_parent_change_aio_context(c, ctx, visited, tran, errp)) {
bdrv_graph_rdunlock_main_loop();
return false;
}
}
QLIST_FOREACH(c, &bs->children, next) {
if (!bdrv_child_change_aio_context(c, ctx, visited, tran, errp)) {
bdrv_graph_rdunlock_main_loop();
return false;
}
}
bdrv_graph_rdunlock_main_loop();
state = g_new(BdrvStateSetAioContext, 1);
*state = (BdrvStateSetAioContext) {
@ -7686,8 +7706,7 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
.bs = bs,
};
/* Paired with bdrv_drained_end in bdrv_set_aio_context_clean() */
bdrv_drained_begin(bs);
assert(bs->quiesce_counter > 0);
tran_add(tran, &set_aio_context, state);
@ -7720,6 +7739,8 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
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);
@ -7733,10 +7754,14 @@ 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);
bdrv_graph_rdunlock_main_loop();
bdrv_drain_all_end();
return 0;
}