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block: Start/end drain on correct AioContext

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bdrv_parent_drained_{begin,end}_single() are supposed to operate on the
parent, not on the child, so they should not attempt to get the context
to poll from the child but the parent instead.  BDRV_POLL_WHILE(c->bs)
does get the context from the child, so we should replace it with
AIO_WAIT_WHILE() on the parent's context instead.

This problem becomes apparent when bdrv_replace_child_noperm() invokes
bdrv_parent_drained_end_single() after removing a child from a subgraph
that is in an I/O thread.  By the time bdrv_parent_drained_end_single()
is called, child->bs is NULL, and so BDRV_POLL_WHILE(c->bs, ...) will
poll the main loop instead of the I/O thread; but anything that
bdrv_parent_drained_end_single_no_poll() may have scheduled is going to
want to run in the I/O thread, but because we poll the main loop, the
I/O thread is never unpaused, and nothing is run, resulting in a
deadlock.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/1215
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20221107151321.211175-4-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Hanna Reitz 2022-11-07 16:13:21 +01:00 committed by Kevin Wolf
parent af5b6ebe5b
commit ace5a161ea
1 changed files with 4 additions and 2 deletions
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6
block/io.c
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@ -71,9 +71,10 @@ static void bdrv_parent_drained_end_single_no_poll(BdrvChild *c,
void bdrv_parent_drained_end_single(BdrvChild *c)
{
int drained_end_counter = 0;
AioContext *ctx = bdrv_child_get_parent_aio_context(c);
IO_OR_GS_CODE();
bdrv_parent_drained_end_single_no_poll(c, &drained_end_counter);
BDRV_POLL_WHILE(c->bs, qatomic_read(&drained_end_counter) > 0);
AIO_WAIT_WHILE(ctx, qatomic_read(&drained_end_counter) > 0);
}
static void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
@ -116,13 +117,14 @@ static bool bdrv_parent_drained_poll(BlockDriverState *bs, BdrvChild *ignore,
void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll)
{
AioContext *ctx = bdrv_child_get_parent_aio_context(c);
IO_OR_GS_CODE();
c->parent_quiesce_counter++;
if (c->klass->drained_begin) {
c->klass->drained_begin(c);
}
if (poll) {
BDRV_POLL_WHILE(c->bs, bdrv_parent_drained_poll_single(c));
AIO_WAIT_WHILE(ctx, bdrv_parent_drained_poll_single(c));
}
}