Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto

Michael Ellerman
2021-08-27T07:53:28Z

Nathan Chancellor <nathan@kernel.org> writes:
> On Thu, Aug 26, 2021 at 11:54:17AM -0700, Nathan Chancellor wrote:
>> On Thu, Aug 26, 2021 at 01:21:39PM +1000, Michael Ellerman wrote:
>> > Nathan Chancellor <nathan@kernel.org> writes:
>> > > On Tue, Apr 13, 2021 at 04:38:10PM +0000, Christophe Leroy wrote:
>> > >> Using asm goto in __WARN_FLAGS() and WARN_ON() allows more
>> > >> flexibility to GCC.
>> > ...
>> > >
>> > > This patch as commit 1e688dd2a3d6 ("powerpc/bug: Provide better
>> > > flexibility to WARN_ON/__WARN_FLAGS() with asm goto") cause a WARN_ON in
>> > > klist_add_tail to trigger over and over on boot when compiling with
>> > > clang:
>> > >
...
>> >
>> > This patch seems to fix it. Not sure if that's just papering over it though.
>> >
>> > diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
>> > index 1ee0f22313ee..75fcb4370d96 100644
>> > --- a/arch/powerpc/include/asm/bug.h
>> > +++ b/arch/powerpc/include/asm/bug.h
>> > @@ -119,7 +119,7 @@ __label_warn_on: \
>> > \
>> > WARN_ENTRY(PPC_TLNEI " %4, 0", \
>> > BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN), \
>> > - __label_warn_on, "r" (x)); \
>> > + __label_warn_on, "r" (!!(x))); \
>> > break; \
>> > __label_warn_on: \
>> > __ret_warn_on = true; \
>> >
>>
>> Thank you for the in-depth explanation and triage! I have gone ahead and
>> created a standalone reproducer that shows this based on the
>> preprocessed file and opened https://bugs.llvm.org/show_bug.cgi?id=51634
>> so the LLVM developers can take a look.
>
> Based on Eli Friedman's comment in the bug, would something like this
> work and not regress GCC? I noticed that the BUG_ON macro does a cast as
> well. Nick pointed out to me privately that we have run into what seems
> like a similar issue before in commit 6c58f25e6938 ("riscv/atomic: Fix
> sign extension for RV64I").

Yes, I read that comment this morning, and then landed at the same fix
via digging through the history of our bug code.

We in fact fixed a similar bug 16 years ago :}

32818c2eb6b8 ("[PATCH] ppc64: Fix issue with gcc 4.0 compiled kernels")

Which is when we first started adding the cast to long.

> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index 1ee0f22313ee..35022667f57d 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -119,7 +119,7 @@ __label_warn_on: \
> \
> WARN_ENTRY(PPC_TLNEI " %4, 0", \
> BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN), \
> - __label_warn_on, "r" (x)); \
> + __label_warn_on, "r" ((__force long)(x))); \
> break; \
> __label_warn_on: \
> __ret_warn_on = true; \

Yeah that fixes the clang build for me.

For GCC it seems to generate the same code in the simple cases:

void test_warn_on_ulong(unsigned long b)
{
WARN_ON(b);
}

void test_warn_on_int(int b)
{
WARN_ON(b);
}

I get:

0000000000000020 <.test_warn_on_ulong>:
20: 0b 03 00 00 tdnei r3,0
24: 4e 80 00 20 blr
28: 60 00 00 00 nop
2c: 60 00 00 00 nop

0000000000000030 <.test_warn_on_int>:
30: 0b 03 00 00 tdnei r3,0
34: 4e 80 00 20 blr

Both before and after the change. So that's good.

For:

void test_warn_on_int_addition(int b)
{
WARN_ON(b+1);
}

Before I get:

0000000000000040 <.test_warn_on_int_addition>:
40: 38 63 00 01 addi r3,r3,1
44: 0b 03 00 00 tdnei r3,0
48: 4e 80 00 20 blr

vs after:

0000000000000040 <.test_warn_on_int_addition>:
40: 38 63 00 01 addi r3,r3,1
44: 7c 63 07 b4 extsw r3,r3
48: 0b 03 00 00 tdnei r3,0
4c: 4e 80 00 20 blr

So there's an extra sign extension we don't need, but I guess we can
probably live with that.

cheers