Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking

[Peter Lieven]

Am 06.03.2013 15:47, schrieb Jeff Cody:
> Commit 9a665b2b made bdrv_truncate() call bdrv_drain_all(), but this breaks
> QCOW images, as well other future image formats (such as VHDX) that may call
> bdrv_truncate(bs->file) from within a read/write operation.  For example, QCOW
> will cause an assert, due to tracked_requests not being empty (since the
> read/write that called bdrv_truncate() is still in progress).
> 
> This modifies the check to only force the bdrv_drain_all() call if the BDS to
> be truncated is not growable, or if we are shrinking the BDS.  Otherwise, if 
> we
> are just growing the file, allow it to happen without forcing a call to
> bdrv_drain_all().

This will actually break iscsi_truncate(). Paolo requested that the 
iscsi_tuncate command
should use sync I/O for rereading the LUN capacity. Using sync I/O is only 
possible if
there are no async requests in-flight.

Looking at the source I have not found a place where bs->growable is set to 0 
for any
block driver, maybe I miss something. Having bs->growable for iSCSI would also 
be ok.

Shouldn't it be possible to call bdrv_drain_all() any time? There are other 
places
where this is called. One I have in mind is e.g. if you cancel an ongoing block 
migration.

Peter

> 
> Signed-off-by: Jeff Cody <address@hidden>
> ---
>  block.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 124a9eb..dce844c 100644
> --- a/block.c
> +++ b/block.c
> @@ -2450,8 +2450,14 @@ int bdrv_truncate(BlockDriverState *bs, int64_t offset)
>      if (bdrv_in_use(bs))
>          return -EBUSY;
>  
> -    /* There better not be any in-flight IOs when we truncate the device. */
> -    bdrv_drain_all();
> +    /* Don't force a drain if we are just growing the file - otherwise,
> +     * using bdrv_truncate() from within a block driver in a read/write
> +     * operation will cause an assert, because bdrv_drain_all() will assert 
> if
> +     * a tracked_request is still in progress. */
> +    if (!bs->growable || offset < bdrv_getlength(bs)) {
> +        /* There better not be any in-flight IOs when we truncate the 
> device. */
> +        bdrv_drain_all();
> +    }
>  
>      ret = drv->bdrv_truncate(bs, offset);
>      if (ret == 0) {
>