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Re: [PATCH v2 05/10] block.c: bdrv_replace_child_noperm: first call ->at
From: |
Emanuele Giuseppe Esposito |
Subject: |
Re: [PATCH v2 05/10] block.c: bdrv_replace_child_noperm: first call ->attach(), and then add child |
Date: |
Wed, 16 Mar 2022 10:16:43 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 |
Unfortunately this patch is not safe: theoretically ->attach can call
bdrv_apply_subtree_drain, and if it polls, will can call a bh that
for example reads the graph, finding it in an inconsistent state, since
it is between the two writes QLIST_INSERT_HEAD(&bs->children, child,
next); and QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
Please ignore it.
This patch could eventually go in the subtree_drain serie, if we decide
to go in that direction.
Emanuele
Am 14/03/2022 um 14:18 schrieb Emanuele Giuseppe Esposito:
> Doing the opposite can make adding the child node to a non-drained node,
> as apply_subtree_drain is only done in ->attach() and thus make
> assert_bdrv_graph_writable fail.
>
> This can happen for example during a transaction rollback (test 245,
> test_io_with_graph_changes):
> 1. a node is removed from the graph, thus it is undrained
> 2. then something happens, and we need to roll back the transactions
> through tran_abort()
> 3. at this point, the current code would first attach the undrained node
> to the graph via QLIST_INSERT_HEAD, and then call ->attach() that
> will take care of restoring the drain with apply_subtree_drain(),
> leaving the node undrained between the two operations.
>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
> block.c | 20 +++++++++++++++-----
> 1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/block.c b/block.c
> index d870ba5393..c6a550f9c6 100644
> --- a/block.c
> +++ b/block.c
> @@ -1434,6 +1434,11 @@ static void bdrv_inherited_options(BdrvChildRole role,
> bool parent_is_format,
> *child_flags = flags;
> }
>
> +/*
> + * Add the child node to child->opaque->children list,
> + * and then apply the drain to the whole child subtree,
> + * so that the drain count matches with the parent.
> + */
> static void bdrv_child_cb_attach(BdrvChild *child)
> {
> BlockDriverState *bs = child->opaque;
> @@ -2889,8 +2894,6 @@ static void bdrv_replace_child_noperm(BdrvChild
> **childp,
> }
>
> if (new_bs) {
> - assert_bdrv_graph_writable(new_bs);
> - QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
>
> /*
> * Detaching the old node may have led to the new node's
> @@ -2901,12 +2904,19 @@ static void bdrv_replace_child_noperm(BdrvChild
> **childp,
> assert(new_bs->quiesce_counter <= new_bs_quiesce_counter);
> drain_saldo += new_bs->quiesce_counter - new_bs_quiesce_counter;
>
> - /* Attach only after starting new drained sections, so that recursive
> - * drain sections coming from @child don't get an extra
> .drained_begin
> - * callback. */
> + /*
> + * First call ->attach() cb.
> + * In child_of_bds case, add child to the parent
> + * (child->opaque) ->children list and if
> + * necessary add missing drains in the child subtree.
> + */
> if (child->klass->attach) {
> child->klass->attach(child);
> }
> +
> + /* Then add child to new_bs->parents list */
> + assert_bdrv_graph_writable(new_bs);
> + QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
> }
>
> /*
>
- [PATCH v2 00/10] block: bug fixes in preparation of AioContext removal, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 03/10] block/io.c: fix bdrv_child_cb_drained_begin invocations from a coroutine, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 07/10] test-bdrv-drain.c: remove test_detach_by_parent_cb(), Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 06/10] test-bdrv-drain.c: adapt test to support additional subtree drains, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 05/10] block.c: bdrv_replace_child_noperm: first call ->attach(), and then add child, Emanuele Giuseppe Esposito, 2022/03/14
- Re: [PATCH v2 05/10] block.c: bdrv_replace_child_noperm: first call ->attach(), and then add child,
Emanuele Giuseppe Esposito <=
- [PATCH v2 01/10] drains: create bh only when polling, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 09/10] child_job_drained_poll: override polling condition only when in home thread, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 10/10] tests/qemu-iotests/030: test_stream_parallel should use auto_finalize=False, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 02/10] bdrv_parent_drained_begin_single: handle calls from coroutine context, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 08/10] tests/unit/test-bdrv-drain.c: graph setup functions can't run in coroutines, Emanuele Giuseppe Esposito, 2022/03/14
- [PATCH v2 04/10] block.c: bdrv_replace_child_noperm: first remove the child, and then call ->detach(), Emanuele Giuseppe Esposito, 2022/03/14