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Re: [Qemu-devel] [PATCH v3 10/12] Dump: add qmp command "query-dump"
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v3 10/12] Dump: add qmp command "query-dump" |
Date: |
Tue, 1 Dec 2015 13:37:37 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 01/12/2015 13:32, Peter Xu wrote:
> On Tue, Dec 01, 2015 at 10:54:48AM +0100, Paolo Bonzini wrote:
>>
>>
>> On 01/12/2015 04:57, Peter Xu wrote:
>>>>> You need a mutex around the reads of ->status and ->written_size.
>>> Could I avoid using mutex here? Let me try to explain what I
>>> thought.
>>>
>>> The concurrency of this should only happen when:
>>>
>>> - detached dump thread is working (dump thread)
>>> - user queries dump status (main thread)
>>>
>>> What the dump thread is doing should be something like:
>>>
>>> - [start dumping]
>>> - inc written_size
>>> - inc written_size
>>> - ...
>>> - inc written_size
>>> - set ->status to COMPLETED|FAILED
>>> - [end dumping]
>>
>> Yes, it's possible but you need to use atomic_mb_read/atomic_mb_set to
>> write ->status. Otherwise a CPU can see the write to ->status before
>> some of the final writes to ->written_size.
>
> Hi, Paolo,
>
> Thanks to point out. However, would it be confusing to use
> atomic_mb_{read|set} rather than directly use smp_rmb() and
> smp_wmb()? Like:
>
> In dump thread:
>
> - inc written_size
> - inc written_size
> - ...
> - inc written_size
> - smp_wmb()
> - atomic_set(status, COMPLETED|FAILED)
>
> In main thread:
>
> - atomic_read(status)
> - smp_rmb()
> - read written_size
>
> What I understand from the doc (seems written by you, thanks :) ) is
> that: atomic_mb_{read|set} is the pair of helper functions for _one_
> specific variable, to make sure its operations are always in order
> as long as we are using atomic_mb_* functions to access it all the
> time. However, in the dump thread case, it's related to read/write
> order of two variables (status and written_size).
atomic_mb_{read,set} does order accesses to the variable against
all other accesses. In this case I'd prefer it to smp_wmb/rmb, because
the writes to written_size are far from the writes to status.
Compare with thread-pool.c:
req->ret = ret;
/* Write ret before state. */
smp_wmb();
req->state = THREAD_DONE;
/* Read state before ret. */
smp_rmb();
/* Schedule ourselves in case elem->common.cb() calls aio_poll() to
* wait for another request that completed at the same time.
*/
qemu_bh_schedule(pool->completion_bh);
elem->common.cb(elem->common.opaque, elem->ret);
It's a matter of taste though. What you wrote above is certainly okay
as well.
Paolo