linux-kernel - Re: [RFC][PATCH 11/12] slub: Replace cmpxchg

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Message-ID: <13ad2c02-c7db-f32f-b085-b92b7dceefa4@zytor.com>
Date:   Mon, 9 Jan 2023 18:09:05 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Heiko Carstens <hca@...ux.ibm.com>, corbet@....net,
        will@...nel.org, boqun.feng@...il.com, mark.rutland@....com,
        catalin.marinas@....com, dennis@...nel.org, tj@...nel.org,
        cl@...ux.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, joro@...tes.org,
        suravee.suthikulpanit@....com, robin.murphy@....com,
        dwmw2@...radead.org, baolu.lu@...ux.intel.com,
        Arnd Bergmann <arnd@...db.de>, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        Andrew Morton <akpm@...ux-foundation.org>, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux.dev,
        linux-arch@...r.kernel.org
Subject: Re: [RFC][PATCH 11/12] slub: Replace cmpxchg_double()

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