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From: Alberto Garcia <berto@igalia.com>
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Date: Tue, 08 Aug 2017 15:01:10 +0200
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Subject: Re: [Qemu-devel] [PATCH v3 4/7] block: convert ThrottleGroup to
 object with QOM
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To: Manos Pitsidianakis <el13635@mail.ntua.gr>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, Kevin Wolf <kwolf@redhat.com>, qemu-block <qemu-block@nongnu.org>

On Wed 02 Aug 2017 12:57:04 PM CEST, Manos Pitsidianakis wrote:
>> At the moment I think throttle_groups_lock isn't strictly needed
>> because incref/decref callers hold the QEMU global mutex anyway.
>>
>> But code accessing throttle_groups still has to be disciplined.
>> Since throttle_groups_lock exists, please use it consistently in all
>> code paths.
>>
>> Alternatively you could remove the lock and document that
>> throttle_groups is protected by the global mutex.  What we can't do
>> is sometimes use throttle_groups_lock and sometimes not use it.
>
> If we use throttle_groups_lock in throttle_group_obj_init() then we
> must give it up in throttle_group_incref() and retake it in
> throttle_group_obj_init(). Maybe indeed it's better to drop
> throttle_groups_lock altogether, since the ThrottleGroup refcounting
> always happens in a QMP or startup/cleanup context.

I checked the code and I also don't see any manipulation of the group
list outside the global mutex, so you can remove throttle_groups_lock,
but please document very clearly that all these calls can only happen
when they are protected by the global mutex.

Berto