Re: [RFC PATCH 0/7] Allow race-free block device handling

Mike Snitzer
2023-02-02T19:57:22Z

On Thu, Feb 02 2023 at 1:41P -0500,
Demi Marie Obenour <demi@invisiblethingslab.com> wrote:

> On Thu, Feb 02, 2023 at 11:50:37AM -0500, Mike Snitzer wrote:
> > On Wed, Jan 25 2023 at 10:33P -0500,
> > Demi Marie Obenour <demi@invisiblethingslab.com> wrote:
> >
> > > This work aims to allow userspace to create and destroy block devices
> > > in a race-free and leak-free way,
> >
> > "race-free and leak-free way" implies there both races and leaks in
> > existing code. You're making claims that are likely very specific to
> > your Xen use-case. Please explain more carefully.
>
> Will do in v2.
>
> > > and to allow them to be exposed to
> > > other Xen VMs via blkback without leaks or races. It’s marked as RFC
> > > for a few reasons:
> > >
> > > - The code has been only lightly tested. It might be unstable or
> > > insecure.
> > >
> > > - The DM_DEV_CREATE ioctl gains a new flag. Unknown flags were
> > > previously ignored, so this could theoretically break buggy userspace
> > > tools.
> >
> > Not seeing a reason that type of DM change is needed. If you feel
> > strongly about it send a separate patch and we can discuss it.
>
> Patch 2/7 is the diskseq change. v2 will contain a revised and tested
> version with a greatly expanded commit message.

I'm aware that 2/7 is where you make the DM change to disallow unknown
flags, what I'm saying is I don't see a reason for that change.

Certainly doesn't look to be a requirement for everything else in that
patch.

So send a separate patch, but I'm inclined to _not_ accept it because
it does potentially break some userspace.

> > > - I have no idea if I got the block device reference counting and
> > > locking correct.
> >
> > Your headers and justifcation for this line of work are really way too
> > terse. Please take the time to clearly make the case for your changes
> > in both the patch headers and code.
>
> I will expand the commit message in v2, but I am not sure what you want
> me to add to the code comments. Would you mind explaining?

Nothing specific about code, was just a general reminder (based on how
terse the 2/7 header was).

Mike