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Subject: Re: [RFC][PATCH 11/12] slub: Replace cmpxchg_double()
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>
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On 1/9/23 14:22, H. Peter Anvin wrote:
>>
>> Another alternative is to try to avoid casting to "u64" as long as
>> humanly possible, and use only "typeof((*ptr))" everywhere. Then when
>> the type actually *is* 128-bit, it all works out fine, because it
>> won't be a pointer. That's the approach the uaccess macros tend to
>> take, and then they hit the reverse issue on clang, where using the
>> "byte register" constraints would cause warnings for non-byte
>> accesses, and we had to do
>>
>>                 unsigned char x_u8__;
>>                 __get_user_asm(x_u8__, ptr, "b", "=q", label);
>>                 (x) = x_u8__;
>>
>> because using '(x)' directly would then warn when 'x' wasn't a
>> char-sized thing - even if that asm case never actually was _used_ for
>> that case, since it was all inside a "switch (sizeof) case 1:"
>> statement.
>>
>>             Linus
> 

> I wrote a crazy macro for dealing with exactly this at one point,
> basically producing the "right type" to cast to. It would need to
> have 128-bit support added to it, but that should be trivial. It is
> called something like int_type() ... not in front of a computer right
> now so can't double check.
Right, it is called __inttype and is defined in 
arch/x86/include/asm/uaccess.h (and, apparently, a few other 
architectures; probably should be centralized.)

It has been rewritten since my first version using a nice little macro 
called __typefits, also would we worth centralizing.

	-hpa