linux-kernel - Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)

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Date:   Tue, 10 Dec 2019 11:15:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, dja@...ens.net,
        elver@...gle.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, christophe.leroy@....fr,
        linux-s390@...r.kernel.org, linux-arch@...r.kernel.org,
        x86@...nel.org, kasan-dev@...glegroups.com,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag
 (topic/kasan-bitops)

On Tue, Dec 10, 2019 at 04:38:54PM +1100, Michael Ellerman wrote:

> Good question, I'll have a look.
> 
> There seems to be confusion about what the type of the bit number is,
> which is leading to sign extension in some cases and not others.

Shiny.

> It looks like the type should be unsigned long?

I'm thinking unsigned makes most sense, I mean, negative bit offsets
should 'work' but that's almost always guaranteed to be an out-of-bound
operation.

As to 'long' vs 'int', I'm not sure, 4G bits is a long bitmap. But I
suppose since the bitmap itself is 'unsigned long', we might as well use
'unsigned long' for the bitnr too.

>   Documentation/core-api/atomic_ops.rst:  void __clear_bit_unlock(unsigned long nr, unsigned long *addr);
>   arch/mips/include/asm/bitops.h:static inline void __clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
>   arch/powerpc/include/asm/bitops.h:static inline void arch___clear_bit_unlock(int nr, volatile unsigned long *addr)
>   arch/riscv/include/asm/bitops.h:static inline void __clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
>   arch/s390/include/asm/bitops.h:static inline void arch___clear_bit_unlock(unsigned long nr,
>   include/asm-generic/bitops/instrumented-lock.h:static inline void __clear_bit_unlock(long nr, volatile unsigned long *addr)
>   include/asm-generic/bitops/lock.h:static inline void __clear_bit_unlock(unsigned int nr,
> 
> So I guess step one is to convert our versions to use unsigned long, so
> we're at least not tripping over that difference when comparing the
> assembly.

Yeah, I'll look at fixing the generic code, bitops/atomic.h and
bitops/non-atomic.h don't even agree on the type of bitnr.