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Re: [Qemu-devel] [PATCH] block: skip initializer BDS on recursive aio co

From: Denis Plotnikov
Subject: Re: [Qemu-devel] [PATCH] block: skip initializer BDS on recursive aio context attachment/detachment
Date: Fri, 15 Feb 2019 08:37:35 +0000

On 08.02.2019 14:03, Kevin Wolf wrote:
> Am 24.01.2019 um 08:48 hat Denis Plotnikov geschrieben:
>> When there is a Backup Block Job running and shutdown command is sent to
>> a guest, the guest crushes due to assert(!bs->walking_aio_notifiers).
>> Call stack:
>> 0  __GI_raise
>> 1  __GI_abort
>> 2  __assert_fail_base
>> 3  __GI___assert_fail
>> 4  bdrv_detach_aio_context (bs=0x55f54d65c000)      <<<
>> 5  bdrv_detach_aio_context (bs=0x55f54fc8a800)
>> 6  bdrv_set_aio_context (bs=0x55f54fc8a800, ...)
>> 7  block_job_attached_aio_context
>> 8  bdrv_attach_aio_context (bs=0x55f54d65c000, ...) <<<
>> 9  bdrv_set_aio_context (bs=0x55f54d65c000)
>> 10 blk_set_aio_context
>> 11 virtio_blk_data_plane_stop
>> 12 virtio_bus_stop_ioeventfd
>> 13 virtio_vmstate_change
>> 14 vm_state_notify (running=0, state=RUN_STATE_SHUTDOWN)
>> 15 do_vm_stop (state=RUN_STATE_SHUTDOWN, send_stop=true)
>> 16 vm_stop (state=RUN_STATE_SHUTDOWN)
>> 17 main_loop_should_exit
>> 18 main_loop
>> 19 main
>> This happens because of "new" context attachment to VM disk bds.
>> When attaching a new context the corresponding aio context handler is
>> called for each of aio_notifiers registered on the VM disk bds context.
>> Among those handlers there is the block_job_attached_aio_context handler
>> which sets a new aio context for the block job bds. When doing so,
>> the old context is detached from all the block job bds children and one of
>> them is the VM disk bds, serving as backing store for the blockjob bds,
>> although the VM disk bds is actually the initializer of that process.
>> Since the VM disk bds is protected with walking_aio_notifiers flag
>> from double processing in recursive calls, the assert fires.
>> The patch fixes the problem by skipping the bds-es in recursive calls
>> which have started attachment/detachment already.
>> Signed-off-by: Denis Plotnikov <address@hidden>
> I'm not sure if this fix is actually correct in more than just your
> special case. It assumes without checking that the nested attach is for
> the same context as the outer attach. (Of course, I'm not sure what you
> would do if they were for different contexts! It's a case to avoid, and
> it should fail very visibly instead of moving nodes to unexpected
> contexts.)
> bdrv_set_aio_context() is pretty messy, so it may still turn out that
> leaving the greater problem unadressed and just fixing what breaks in
> practice is what we need to do.
> But did you consider just forbidding nested bdrv_set_aio_context()
> instead? Essentially, instead of your checks you would assert
> !walking_aio_notifiers. This would simplify the interface instead of
> adding more complexity.
> If I understand the code correctly, at the time of the nested call, the
> respective nodes have already been moved to the new context, so avoiding
> the nested calls might be as easy as adding a shortcut to
> bdrv_set_aio_context() for this case:
>      if (ctx == new_context) {
>          return;
>      }
Yes, this is probably the best solution. I'll send the patch shortly.
> Most importantly, though, please write a test case for this. We will
> probably need to rework bdrv_set_aio_context() fundamentally (as Berto
> recently noticed), and we will want to avoid regressing on this bug.
Will do
> Kevin



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