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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>, dja@axtens.net,
        elver@google.com, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, christophe.leroy@c-s.fr,
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        x86@kernel.org, kasan-dev@googlegroups.com,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)
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Date:   Wed, 11 Dec 2019 11:29:16 +1100
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Peter Zijlstra <peterz@infradead.org> writes:
> 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.

Yeah I agree.

> 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.

4G is a lot of bits, but it's not *that* many.

eg. If we had a bit per 4K page on a 32T machine that would be 8G bits.

So unsigned long seems best.

>>   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.

Thanks.

cheers