Re: [Qemu-devel] [PATCH 2/2] sheepdog: improve error handling for a case of failed lock

[Hitoshi Mitake]

At Fri, 8 Aug 2014 13:31:39 +0800,
Liu Yuan wrote:
> 
> On Thu, Aug 07, 2014 at 04:28:40PM +0900, Hitoshi Mitake wrote:
> > Recently, sheepdog revived its VDI locking functionality. This patch
> > updates sheepdog driver of QEMU for this feature:
> > 
> > 1. Improve error message when QEMU fails to acquire lock of
> > VDI. Current sheepdog driver prints an error message "VDI isn't
> > locked" when it fails to acquire lock. It is a little bit confusing
> > because the mesage says VDI isn't locked but it is actually locked by
> > other VM. This patch modifies this confusing message.
> > 
> > 2. Change error code for a case of failed locking. -EBUSY is a
> > suitable one.
> > 
> > Reported-by: Valerio Pachera <address@hidden>
> > Cc: Kevin Wolf <address@hidden>
> > Cc: Stefan Hajnoczi <address@hidden>
> > Cc: Liu Yuan <address@hidden>
> > Cc: MORITA Kazutaka <address@hidden>
> > Signed-off-by: Hitoshi Mitake <address@hidden>
> > ---
> >  block/sheepdog.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/block/sheepdog.c b/block/sheepdog.c
> > index 36f76f0..0b3f86d 100644
> > --- a/block/sheepdog.c
> > +++ b/block/sheepdog.c
> > @@ -1112,9 +1112,13 @@ static int find_vdi_name(BDRVSheepdogState *s, const 
> > char *filename,
> >  
> >      if (rsp->result != SD_RES_SUCCESS) {
> >          error_setg(errp, "cannot get vdi info, %s, %s %" PRIu32 " %s",
> > +                   rsp->result == SD_RES_VDI_NOT_LOCKED ?
> 
> I'm puzzled by this check.
> 
> we use SD_RES_VDI_LOCKED to indicate vid is already locked, no?

We use SD_RES_VDI_NOT_LOCKED for indicating locking by this VM fails.

> 
> > +                   "VDI is already locked by other VM" :
> >                     sd_strerror(rsp->result), filename, snapid, tag);
> >          if (rsp->result == SD_RES_NO_VDI) {
> >              ret = -ENOENT;
> > +        } else if (rsp->result == SD_RES_VDI_NOT_LOCKED) {
> > +            ret = -EBUSY;
> >          } else {
> >              ret = -EIO;
> >          }
> 
> It is better to use switch case to handle the result.

using switch statement in this case only increases a number of lines
of code:

Current change:
        if (rsp->result == SD_RES_NO_VDI) {
            ret = -ENOENT;
        } else if (rsp->result == SD_RES_VDI_NOT_LOCKED) {
...

Change with switch:
        switch (rsp->result) {
            case SD_RES_NO_VDI:
            ret = -ENOENT;
            break;
            case SD_RES_VDI_NOT_LOCKED:
...

The change with switch statement requires one more line for break;. I
think if statement is suitable for this case.

Thanks,
Hitoshi