linux-kernel - Re: Linux 6.3-rc3

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Date:   Mon, 20 Mar 2023 12:22:15 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-toolchains@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: Linux 6.3-rc3

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@...nel.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