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Re: [Qemu-devel] [PATCH v3 2/8] block: add basic backup support to block

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v3 2/8] block: add basic backup support to block driver
Date: Tue, 21 May 2013 17:11:57 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 21/05/2013 15:34, Stefan Hajnoczi ha scritto:
> /**
>  * hbitmap_iter_init:
> [...]
>  * position of the iterator is also okay.  However, concurrent resetting of
>  * bits can lead to unexpected behavior if the iterator has not yet reached
>  * those bits. 
>  */ 
> void hbitmap_iter_init(HBitmapIter *hbi, const HBitmap *hb, uint64_t first);
> This worries me.  We would initialize the bitmap to all 1s.  Backing up
> a cluster resets the bit.  But the documentation says it is not safe to
> reset bits while iterating?

Hmm, right.  But do we need the bitmap at all?  We can just use
bdrv_is_allocated like bdrv_co_do_readv does.

The code has a comment:

> +        /* immediately set bitmap (avoid coroutine race) */
> +        hbitmap_set(job->bitmap, start, 1);

but wouldn't this be avoided anyway, because of the mutual exclusion
between overlapping requests?


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