Re: [Qemu-devel] [PATCH v2] block/file-posix: ignore fail on unlock bytes

[Kevin Wolf]

Am 29.03.2019 um 11:55 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 29.03.2019 13:12, Kevin Wolf wrote:
> > Am 29.03.2019 um 10:53 hat Vladimir Sementsov-Ogievskiy geschrieben:
> >> 28.03.2019 21:40, Kevin Wolf wrote:
> >>> Am 28.03.2019 um 08:21 hat Vladimir Sementsov-Ogievskiy geschrieben:
> >>>> bdrv_replace_child() calls bdrv_check_perm() with error_abort on
> >>>> loosening permissions. However file-locking operations may fail even
> >>>> in this case, for example on NFS. And this leads to Qemu crash.
> >>>>
> >>>> Let's ignore such errors, as we do already on permission update commit
> >>>> and abort.
> >>>>
> >>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> >>>
> >>> I think this would better be fixed in block.c code so that unlock never
> >>> fails for any block driver.
> >>
> >> Hmm. We now only have one .bdrv_check_perm handler - raw_check_perm. And
> >> in this particular case, yes, the only thing we can do is ignoring error
> >> and do not fail on loosening permissions..
> >>
> >> If we have more drivers with this callback, what should be the common
> >> behavior?
> >>
> >> Do you propose to ignore .bdrv_check_perm errors in common case?
> >>
> >> Isn't it better to require, that .bdrv_check_perm handler do not fail on
> >> loosening permissions, and abort if it fail in this case, like it actually
> >> works after this patch?
> > 
> > Maybe an assertion in the common code is actually better, yes.
> > 
> > I do think that the common behaviour we want is to ignore
> > .bdrv_check_perm errors for unlock, but it might be surprising for
> > drivers that .bdrv_set_perm is called when .bdrv_check_perm failed. So
> > we need the drivers to be aware of the problem anyway.
> > 
> > Back to your patch: Why do you need to call raw_check_lock_bytes() in
> > the unlock case? We don't acquire any new permissions and hold the locks
> > for everything, so nobody else should have taken a conflicting lock.
> > 
> 
> Hmm.. it check not same arguments, so I didn't drop it as
> raw_apply_lock_bytes.

Not exactly the same, but a subset of the old ones.

> On the other hand, the only meaning of this raw_check_lock_bytes, is
> that we'll print error if it come (when it should not).
> 
> Seems OK for me to drop it too and just return 0 immediately.

Okay, so the plan for v3 is to drop the raw_check_lock_bytes() call
here, and add an assertion in block.c, right?

Kevin