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 2018-06-26 21:30, Max Reitz wrote:
> 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.

Scratch that, it seems that it's not just tmpfs but just breakage in
general.  I suppose that's better.

Max


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