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Message-ID: <576B3588.1070806@cn.fujitsu.com>
Date: Thu, 23 Jun 2016 09:04:08 +0800
From: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] blockjob: assert(cb) in the entry
 functions of blockjob
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To: Eric Blake <eblake@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, qemu devel <qemu-devel@nongnu.org>, Jeff Cody <jcody@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Max Reitz <mreitz@redhat.com>

On 06/23/2016 01:31 AM, Eric Blake wrote:
> On 06/22/2016 04:19 AM, Paolo Bonzini wrote:
>>
>>
>> On 22/06/2016 12:12, Changlong Xie wrote:
>>>
>>> commit/mirror/stream/backup use block_job_create(..., cb,..) to create
>>> relevant blockjob. When they finished, these jobs will invoke
>>> block_job_completed, then invoke job->cb() unconditionally. So i think
>>> we need this to avoid segment fault. Actually backup has implemented this.
>>
>> So this suggests that the right place to put the assertion would be
>> block_job_create.  But it's even better to add a
>>
>> #define QEMU_NONNULL __attribute__((__nonnull__))
>>
>> to include/qemu/compiler.h and declare the arguments as non-null.
>
> Or alternatively fix things to only invoke job->cb() if it is non-NULL,
> so that callers don't have to pass in a no-op callback just to appease a
> non-NULL attribute.
>

Is there any reason, that we should invoke job->cb() unconditionally 
without nonnull check? There is no relevant clue in the historical 
commit messages. If yes, i prefer paolo's suggestion; otherwise eric's 
solution is better.

Thanks
	-Xie