Re: Linux 6.3-rc3

Nathan Chancellor
2023-03-20T19:30:24Z

On Mon, Mar 20, 2023 at 11:53:37AM -0700, Nathan Chancellor wrote:
> On Mon, Mar 20, 2023 at 11:26:17AM -0700, Linus Torvalds wrote:
> > On Mon, Mar 20, 2023 at 11:05 AM Nathan Chancellor <nathan@kernel.org> wrote:
> > >
> > > On the clang front, I am still seeing the following warning turned error
> > > for arm64 allmodconfig at least:
> > >
> > > drivers/gpu/host1x/dev.c:520:6: error: variable 'syncpt_irq' is uninitialized when used here [-Werror,-Wuninitialized]
> > > if (syncpt_irq < 0)
> > > ^~~~~~~~~~
> >
> > Hmm. I do my arm64 allmodconfig builds with gcc, and I'm surprised
> > that gcc doesn't warn about this.
>
> Perhaps these would make doing allmodconfig builds with clang more
> frequently less painful for you?
>
> https://lore.kernel.org/llvm/20230319235619.GA18547@dev-arch.thelio-3990X/
>
> > That syncpt_irq thing isn't written to anywhere, so that's pretty egregious.
> >
> > We use -Wno-maybe-uninitialized because gcc gets it so wrong, but
> > that's different from the "-Wuninitialized" thing (without the
> > "maybe").
> >
> > I've seen gcc mess this up when there is one single assignment,
> > because then the SSA format makes it *so* easy to just use that
> > assignment out-of-order (or unconditionally), but this case looks
> > unusually clear-cut.
> >
> > So the fact that gcc doesn't warn about it is outright odd.
> >
> > > If that does not come to you through other means before -rc4, could you
> > > just apply it directly so that I can stop applying it to our CI? :)
> >
> > Bah. I took it now, there's no excuse for that thing.
>
> Thanks!
>
> > Do we have any gcc people around that could explain why gcc failed so
> > miserably at this trivial case?
>
> Cc'ing linux-toolchains. The start of the thread is here:
>
> https://lore.kernel.org/CAHk-=wgSqpdkeJBb92M37JNTdRQJRnRUApraHKE8uGHTqQuu2Q@mail.gmail.com/
>
> The problematic function before the fix is here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/host1x/dev.c?id=3d3699bde4b043eea17993e4e76804a8128f0fdb#n487
>
> I will see if I have some cycles to try and reduce something out for the
> GCC folks.

While setting up the reduction, I noticed that there is an instance of
-Wmaybe-uninitialized at this site. Seems odd that it is not sure, I will
reduce on that.

../drivers/gpu/host1x/dev.c: In function 'host1x_probe':
../drivers/gpu/host1x/dev.c:520:12: error: 'syncpt_irq' may be used uninitialized [-Werror=maybe-uninitialized]
520 | if (syncpt_irq < 0)
| ^
../drivers/gpu/host1x/dev.c:490:13: note: 'syncpt_irq' was declared here
490 | int syncpt_irq;
| ^~~~~~~~~~
cc1: all warnings being treated as errors

Cheers,
Nathan