Re: [Qemu-devel] [PATCH 16/19] block: protect modification of dirty bitmaps with a mutex

[Paolo Bonzini]

On 27/06/2017 16:20, Vladimir Sementsov-Ogievskiy wrote:
> I'm likely not right, but for me introducing this mutex looks like dirty
> bitmaps may be accessed from concurrent threads. So for me it looks
> strange that only accessors are protected by the mutex and not the whole
> transactions.

There must be something else protecting the whole transaction.

> One example I've wrote above, other examples from code: are
> qmp_block_dirty_bitmap** functions:
> 
> bdrv_create_dirty_bitmap() {
> 
>    bdrv_find_dirty_bitmap()
> 
>    ....
> 
> bdrv_dirty_bitmaps_lock(bs);
>    QLIST_INSERT_HEAD(&bs->dirty_bitmaps, bitmap, list);
>    bdrv_dirty_bitmaps_unlock(bs);
> 
> }
> 
> - we protect inserting into list from other threads, but what prevent
> creating bitmap with the same name from other thread after
> bdrv_find_dirty_bitmap() and before bdrv_dirty_bitmaps_lock() ?

It's like a read-write lock.

The write side is invoked under the 'big QEMU lock' so there cannot be
two concurrent writes.

A bitmap can be written to after bdrv_find_dirty_bitmap returns, but
only if _you_ tell another thread about the bitmap you've just created.
If that doesn't happen, the bitmap cannot change.  And it can also
disappear because _your_ thread is the one with the big QEMU lock.

Paolo