Re: deadlock when using iothread during backup_clean()

[Fiona Ebner <f.ebner@proxmox.com>]

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