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Date: Fri, 15 Feb 2019 17:57:55 +0100
From: Kevin Wolf <kwolf@redhat.com>
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References: <20190215130325.5294-1-dplotnikov@virtuozzo.com>
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In-Reply-To: <20190215130325.5294-1-dplotnikov@virtuozzo.com>
Subject: Re: [Qemu-devel] [PATCH v2] block: don't set the same context
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To: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: mreitz@redhat.com, eblake@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org, den@virtuozzo.com

Am 15.02.2019 um 14:03 hat Denis Plotnikov geschrieben:
> Adds a fast path on aio context setting preventing
> unnecessary context setting routine.
> Also, it prevents issues with cyclic walk of child
> bds-es appeared because of registering aio walking
> 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 can happen 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.
> 
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>

Thanks, applied to the block branch.

Kevin