Re: [Qemu-block] [PATCH] block: Don't lock /dev/null and /dev/zero automatically

[Fam Zheng]

On Sun, Jul 22, 2018 at 5:08 AM Max Reitz <address@hidden> wrote:
>
> On 2018-07-19 05:41, Fam Zheng wrote:
> > On my Fedora 28, /dev/null is locked by some other process (couldn't
> > inspect it due to the current lslocks limitation), so iotests 226 fails
> > with some unexpected image locking errors because it uses qemu-io to
> > open it.
> >
> > Actually it's safe to not use any lock on /dev/null or /dev/zero.
> >
> > Signed-off-by: Fam Zheng <address@hidden>
> > ---
> >  block/file-posix.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index 60af4b3d51..8bf034108a 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -503,7 +503,12 @@ static int raw_open_common(BlockDriverState *bs, QDict 
> > *options,
> >          s->use_lock = false;
> >          break;
> >      case ON_OFF_AUTO_AUTO:
> > -        s->use_lock = qemu_has_ofd_lock();
> > +        if (!strcmp(filename, "/dev/null") ||
> > +                   !strcmp(filename, "/dev/zero")) {
>
> I’m not sure I like a strcmp() based on filename (though it should work
> for all of the cases where someone would want to specify either of those
> for a qemu block device).  Isn’t there some specific major/minor number
> we can use to check for these two files?  Or are those Linux-specific?

Yeah, I guess major/minor numbers are Linux-specific.

>
> I could also imagine just not locking any host_device, but since people
> do use qcow2 immediately on block devices, maybe we do want to lock them.

That's right.

>
> Finally, if really all you are trying to do is to make test code easier,
> then I don’t know exactly why.  Just disabling locking in 226 shouldn’t
> be too hard.

The tricky thing is in remembering to do that for future test cases.
My machine seems to be somehow different than John's so that my 226
cannot lock /dev/null, but I'm not sure that is the case for other
releases, distros or system instances.

Fam

>
> Max
>
> > +            s->use_lock = false;
> > +        } else {
> > +            s->use_lock = qemu_has_ofd_lock();
> > +        }
> >          break;
> >      default:
> >          abort();
> >
>
>