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Re: [Qemu-devel] [PATCH 16/19] block: protect modification of dirty bitm


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 16/19] block: protect modification of dirty bitmaps with a mutex
Date: Tue, 27 Jun 2017 15:58:15 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0


On 27/06/2017 15:51, Vladimir Sementsov-Ogievskiy wrote:
>>>
>>> bdrv_create_dirty_bitmap(...)
>>>
>>> bdrv_dirty_bitmaps_lock(bs)
>>>
>>> bitmap  = bdrv_find_dirty_bitmap(bs, name)
>>>
>>> <some changes>
>>>
>>> bdrv_dirty_bitmaps_unlock(bs)
>>>
>>> - because we can't now trust the pointer returned by
>>> bdrv_create_dirty_bitmap, as it releases bitmap lock before return.
>> If you have the big QEMU lock (you do if you are in the QEMU monitor),
>> you are protected from changes to the list of bitmaps.
>>
>> Paolo
> 
> but you wrote "Writing to the list requires the BQL _and_ the
> dirty_bitmap_mutex".
> 
> should it be BQL only?

bdrv_dirty_bitmaps_lock/unlock is (should be) called by the functions
that write to the list.  I hope I can post the patch today already.

Paolo



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