Re: [PATCH v2 0/2] Lock and Pointer guards

Ian Lance Taylor
2023-05-29T21:27:47Z

On Mon, May 29, 2023 at 12:04 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, May 29, 2023 at 8:09 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > On Sat, May 27, 2023 at 9:18 PM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > > It's also an example of something people need to be aware of: the
> > > auto-releasing is not ordered. That may or may not be a problem. It's
> > > not a problem for two identical locks, but it very much *can* be a
> > > problem if you mix different locks.
> >
> > It is guaranteed. It would be nice to have it documented, but if you
> > look at the intermediate representation of this simple example:
>
> Well, I can see that it might be doing that reverse ordering in
> practice, but for the life of me, I can't actually find anything that
> says it is guaranteed.
>
> Google did find me one blog post by Ian Lance Taylor from 2008 that
> said that yes, each __cleanup__ attribute basically creates its own
> little scope, and that the cleanup in reverse declaration order is
> thus guaranteed.
>
> Let's add Ian to the cc, partly to confirm it wasn't just a random
> implementation detail, but also partly to perhaps ask him to get
> somebody to document it.
>
> Because if it's not documented, how do we know that the clang
> implementation of that attribute then ends up also guaranteeing the
> reverse order cleanup, even if gcc might guarantee it?
>
> I *suspect* - but cannot find any guarantees - that it's going to
> match C++ destructors, and you probably end up pretty much always
> having to deal with these cleanup functions in reverse order, so it
> all sounds very likely to me.
>
> And maybe it's even documented somewhere that neither of us could find.
>
> Anyway, I do like the option to use cleanup functions, but I think
> we'd better make sure, since we really may require nesting for locks
> (even if in many cases it won't matter).
>
> Ian? Any chance you can either point us at documentation, or maybe
> just confirm it, and hopefully make said documentation happen?

It was a while ago, but I expect that I was just thinking of the
implementation. I agree that the documentation could be clearer. I
filed https://gcc.gnu.org/PR110029.

Ian