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Date: Wed, 12 Oct 2016 08:42:50 +0800
From: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
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Subject: Re: [Qemu-devel] [Questions] NBD issue or CoMutex->holder issue?
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu devel <qemu-devel@nongnu.org>, qemu block <qemu-block@nongnu.org>, Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Max Reitz <mreitz@redhat.com>, Fam Zheng <famz@redhat.com>, Eric Blake <eblake@redhat.com>, Wen Congyang <wency@cn.fujitsu.com>

On 10/11/2016 06:47 PM, Paolo Bonzini wrote:
>> the free_sema->queue head, so set free_sema->holder as
>> >revelant coroutine.
> NBD is using the CoMutex in a way that wasn't anticipated.  The simplest
> fix is to change it to CoQueue, which is like a condition variable.
> Instead of locking if in_flight >= MAX_NBD_REQUESTS - 1, wait on the
> queue while in_flight == MAX_NBD_REQUESTS.  Instead of unlocking, use
> qemu_co_queue_next to wake up one request.
>

Thanks for your explanation! will send out a patch later.


Thanks
	-Xie

> Thanks for the report!
>
> Paolo
>
>> >For example if there are N(N=26 and MAX_NBD_REQUESTS=16) nbd write
>> >requests, so we'll invoke nbd_client_co_pwritev 26 times.
>> >time     request No   Actions
>> >1         1           in_flight=1, Coroutine=C1
>> >2         2           in_flight=2, Coroutine=C2