Re: [Qemu-devel] [PATCH for 2.9 v3 10/10] block: Fix bdrv_co_flush early return

[Fam Zheng <famz@redhat.com>]

On Tue, 04/25 17:16, Kevin Wolf wrote:
> Am 10.04.2017 um 17:05 hat Fam Zheng geschrieben:
> > bdrv_inc_in_flight and bdrv_dec_in_flight are mandatory for
> > BDRV_POLL_WHILE to work, even for the shortcut case where flush is
> > unnecessary. Move the if block to below bdrv_dec_in_flight, and BTW fix
> > the variable declaration position.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/io.c | 16 +++++++++-------
> >  1 file changed, 9 insertions(+), 7 deletions(-)
> > 
> > diff --git a/block/io.c b/block/io.c
> > index 00e45ca..bae6947 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -2278,16 +2278,17 @@ static void coroutine_fn bdrv_flush_co_entry(void *opaque)
> >  
> >  int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
> >  {
> > -    int ret;
> > -
> > -    if (!bs || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs) ||
> > -        bdrv_is_sg(bs)) {
> > -        return 0;
> > -    }
> > +    int current_gen;
> > +    int ret = 0;
> >  
> >      bdrv_inc_in_flight(bs);
> 
> As Coverity points out, we're now using bs...
> 
> > -    int current_gen = bs->write_gen;
> > +    if (!bs || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs) ||
> 
> ...before doing the NULL check.
> 
> I'm not sure if we even need to have a NULL check here, but we would have
> to check all callers to make sure that it's unnecessary. Before commit
> 29cdb251, it only checked bs->drv and I don't see how that commit
> introduced a NULL caller, but maybe one was added later.
> 
> In any case, bdrv_co_flush() needs a fix, either remove the NULL check
> or do it first.

After auditing the callers and knowing the fact that the above
bdrv_inc_in_flight didn't cause a problem, I think removing the NULL check is
fine.

I'll send a patch.

Thanks.

Fam

> 
> > +        bdrv_is_sg(bs)) {
> > +        goto early_exit;
> > +    }
> 
> Kevin