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Re: [Qemu-devel] [PATCH v2] block: fix QEMU crash with scsi-hd and drive
From: |
Greg KH |
Subject: |
Re: [Qemu-devel] [PATCH v2] block: fix QEMU crash with scsi-hd and drive_del |
Date: |
Thu, 24 May 2018 17:02:12 +0200 |
User-agent: |
Mutt/1.10.0 (2018-05-17) |
On Thu, May 24, 2018 at 11:09:47AM +0200, 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. This patch hence adds
> a drained section to bdrv_detach_child(), so that we're sure
> all requests have been drained when blk->root is freed.
>
> Signed-off-by: Greg Kurz <address@hidden>
> ---
> v2: - drain I/O requests when detaching the BDS (Stefan, Paolo)
> ---
> block.c | 4 ++++
> 1 file changed, 4 insertions(+)
<formletter>
This is not the correct way to submit patches for inclusion in the
stable kernel tree. Please read:
https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.
</formletter>