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

[Kevin Wolf]

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.

Kevin