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Re: deadlock when using iothread during backup_clean()
From: |
Fiona Ebner |
Subject: |
Re: deadlock when using iothread during backup_clean() |
Date: |
Tue, 5 Sep 2023 13:25:40 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.0 |
Am 05.09.23 um 12:01 schrieb Fiona Ebner:
>
> Can we assume block_job_remove_all_bdrv() to always hold the job's
> AioContext? And if yes, can we just tell bdrv_graph_wrlock() that it
> needs to release that before polling to fix the deadlock?
>
I tried a doing something similar as a proof-of-concept
> diff --git a/blockjob.c b/blockjob.c
> index 58c5d64539..1a696241a0 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -198,19 +198,19 @@ void block_job_remove_all_bdrv(BlockJob *job)
> * one to make sure that such a concurrent access does not attempt
> * to process an already freed BdrvChild.
> */
> - bdrv_graph_wrlock(NULL);
> while (job->nodes) {
> GSList *l = job->nodes;
> BdrvChild *c = l->data;
>
> job->nodes = l->next;
>
> + bdrv_graph_wrlock(c->bs);
> bdrv_op_unblock_all(c->bs, job->blocker);
> bdrv_root_unref_child(c);
> + bdrv_graph_wrunlock();
>
> g_slist_free_1(l);
> }
> - bdrv_graph_wrunlock();
> }
and while it did get slightly further, I ran into another deadlock with
> #0 0x00007f1941155136 in __ppoll (fds=0x55992068fb20, nfds=2,
> timeout=<optimized out>, sigmask=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:42
> #1 0x000055991c6a1a3f in qemu_poll_ns (fds=0x55992068fb20, nfds=2,
> timeout=-1) at ../util/qemu-timer.c:339
> #2 0x000055991c67ed6c in fdmon_poll_wait (ctx=0x55991f058810,
> ready_list=0x7ffda8c987b0, timeout=-1) at ../util/fdmon-poll.c:79
> #3 0x000055991c67e6a8 in aio_poll (ctx=0x55991f058810, blocking=true) at
> ../util/aio-posix.c:670
> #4 0x000055991c50a763 in bdrv_graph_wrlock (bs=0x0) at
> ../block/graph-lock.c:145
> #5 0x000055991c4daf85 in bdrv_close (bs=0x55991fff2f30) at ../block.c:5166
> #6 0x000055991c4dc050 in bdrv_delete (bs=0x55991fff2f30) at ../block.c:5606
> #7 0x000055991c4df205 in bdrv_unref (bs=0x55991fff2f30) at ../block.c:7173
> #8 0x000055991c4fb8ca in bdrv_cbw_drop (bs=0x55991fff2f30) at
> ../block/copy-before-write.c:566
> #9 0x000055991c4f9685 in backup_clean (job=0x55992016d0b0) at
> ../block/backup.c:105