Re: [Qemu-devel] [PATCH v4] block: fix QEMU crash with scsi-hd and drive_del

[Max Reitz <mreitz@redhat.com>]

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On 2018-05-28 14:03, Greg Kurz wrote:
> Removing a drive with drive_del while it is being used to run an I/O
> intensive workload can cause QEMU to crash.
> 
> An AIO flush can yield at some point:
> 
> blk_aio_flush_entry()
>  blk_co_flush(blk)
>   bdrv_co_flush(blk->root->bs)
>    ...
>     qemu_coroutine_yield()
> 
> and let the HMP command to run, free blk->root and give control
> back to the AIO flush:
> 
>     hmp_drive_del()
>      blk_remove_bs()
>       bdrv_root_unref_child(blk->root)
>        child_bs = blk->root->bs
>        bdrv_detach_child(blk->root)
>         bdrv_replace_child(blk->root, NULL)
>          blk->root->bs = NULL
>         g_free(blk->root) <============== blk->root becomes stale
>        bdrv_unref(child_bs)
>         bdrv_delete(child_bs)
>          bdrv_close()
>           bdrv_drained_begin()
>            bdrv_do_drained_begin()
>             bdrv_drain_recurse()
>              aio_poll()
>               ...
>               qemu_coroutine_switch()
> 
> and the AIO flush completion ends up dereferencing blk->root:
> 
>   blk_aio_complete()
>    scsi_aio_complete()
>     blk_get_aio_context(blk)
>      bs = blk_bs(blk)
>  ie, bs = blk->root ? blk->root->bs : NULL
>             ^^^^^
>             stale
> 
> The problem is that we should avoid making block driver graph
> changes while we have in-flight requests. Let's drain all I/O
> for this BB before calling bdrv_root_unref_child().
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> v4: - call blk_drain() in blk_remove_bs() (Kevin)
> 
> v3: - start drained section before modifying the graph (Stefan)
> 
> v2: - drain I/O requests when detaching the BDS (Stefan, Paolo)
> ---
>  block/block-backend.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 89f47b00ea24..bee1f0e41461 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -768,6 +768,11 @@ void blk_remove_bs(BlockBackend *blk)
>  
>      blk_update_root_state(blk);
>  
> +    /* bdrv_root_unref_child() will cause blk->root to become stale and may
> +     * switch to a completion coroutine later on. Let's drain all I/O here
> +     * to avoid that and a potential QEMU crash.
> +     */
> +    blk_drain(blk);
>      bdrv_root_unref_child(blk->root);
>      blk->root = NULL;
>  }

For some reason, this patch breaks iotest 083 (with -nbd) on tmpfs for me.

Only on tmpfs, though, so it's probably not going to be just a simple
reference output fix.

Max


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