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Re: [Qemu-block] [Qemu-devel] [PATCH v9 03/13] block/dirty-bitmap: add _

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v9 03/13] block/dirty-bitmap: add _locked version of bdrv_reclaim_dirty_bitmap
Date: Mon, 12 Feb 2018 20:30:47 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

18.01.2018 13:09, Paolo Bonzini wrote:
On 18/01/2018 10:55, Vladimir Sementsov-Ogievskiy wrote:
Most functions that looks at the list are "called with BQL taken".
Functions that write to the list are "called with BQL taken" and call
bdrv_dirty_bitmaps_lock/bdrv_dirty_bitmaps_unlock themselves.
Paolo, could you please explain about bitmap locking in more details?
Why do we need mutexes?
We have three cases:

1) monitor creates and destroy bitmaps.

2) monitor also has to read the list.  We know it operates with BQL.

3) users such as mirror.c create a dirty bitmap in the monitor command
(under BQL), but they can operate without BQL in a separate iothread so
we create a separate lock (bitmap->mutex).

While in the second and third case, bitmaps cannot disappear.  So in the
first case you operate with BQL+dirty bitmap mutex.  The result is that
you lock out both the second and the third case while creating and
destroying bitmaps.

Why do we do not need them
on read from the bitmap, only on write?
Indeed, reading the bitmap also requires taking the lock.  So
s/Modifying/Accessing/ in that comment.


So, finally, the whole thing is:

1. any access to dirty_bitmaps list needs BQL or dirty_bitmap_mutex
2. bitmap creation or removing needs both BQL and dirty_bitmap_mutex


and one more question:
Do we really have users, which accesses dirty bitmaps with only BQL? query-block uses dirty_bitmap_mutex..

Best regards,

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