Re: [Qemu-devel] [PATCH] use bdrv_flush to provide barrier semantic in block/vdi.c for metadata updates

[Max Reitz <mreitz@redhat.com>]

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CC-ing qemu-block and Stefan Weil (maintainer of vdi).

On 06.05.2015 19:23, phoeagon wrote:
> Thanks for your input.
>
> So I changed it to:
> 1. Only call bdrv_flush when bdrv_pwrite was successful
> 2. Only if bdrv_flush was unsuccessful that the return value of 
> vdi_co_write is updated.

One of both is enough. Both are too much. :-)

It is indeed correct, technically (because ret is 0 before the 
bdrv_write()), but it's too verbose. (See below)

> In this way we try to avoid messing up any potential return value 
> checks possible while still propagating bdrv_flush errors.
> That return value was a catch and I admit I'm no pro with the return 
> value convention in QEMU. bdrv_pwrite doesn't return the same value as 
> bdrv_pwrite_sync I assume (they do return negative values when fail, 
> but different values when successful)

It doesn't really matter, I think. Returning any non-negative value from 
vdi_co_write() should be enough to signal success.

> ---
>
>
> Signed-off-by: Zhe Qiu <address@hidden>
>
> From 19b2fabbe00765b418362d8c1891f266091621f3 Mon Sep 17 00:00:00 2001
> From: phoeagon <address-hidden>
> Date: Thu, 7 May 2015 01:09:38 +0800
> Subject: [PATCH] block/vdi: Use bdrv_flush after metadata updates
>
> In reference to 
> b0ad5a455d7e5352d4c86ba945112011dbeadfb8~078a458e077d6b0db262c4b05fee51d01de2d1d2, 
> metadata writes to qcow2/cow/qcow/vpc/vmdk are all synced prior to 
> succeeding writes.
>
> ---
>  block/vdi.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/block/vdi.c b/block/vdi.c
> index 5d09b36..54a5fa8 100644
> --- a/block/vdi.c
> +++ b/block/vdi.c
> @@ -713,7 +713,11 @@ static int vdi_co_write(BlockDriverState *bs,
>          logout("will write %u block map sectors starting from entry 
> %u\n",
>                 n_sectors, bmap_first);
>          ret = bdrv_write(bs->file, offset, base, n_sectors);
> +        if (!(ret < 0)) {
> +            int flush_ret = bdrv_flush(bs->file);
> +            if (flush_ret < 0)
> +                ret = flush_ret;
> +        }

I think bdrv_write() always returns 0 on success. In any case, it's fine 
for vdi_co_write() to return 0 on success (which is what bdrv_flush() 
returns), so shorting these four lines to "ret = bdrv_flush(bs->file);" 
is enough.

The patch is correct, though, so if you want to leave it as it is, all 
you need to do is bring it into proper form 
(http://wiki.qemu.org/Contribute/SubmitAPatch).

The previous version was nearly right, except for the things I 
mentioned: The subject needs to start with the part of qemu the patch is 
targeting (in this case "block/vdi: " or simply "vdi: "), the 
Signed-off-by needs to contain your name (or any alias you desire) and 
your email address, and comments for the patch should be separated from 
the actual commit message by "---".

Finally, for sending the next version, please change the "[PATCH]" in 
the subject to "[PATCH v3]" in order to indicate that it will be version 
3 of this patch.

Thanks!

Max

>      }
>      return ret;
> -- 
> 2.4.0


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