linux-kernel - Re: [GIT PULL] percpu fix for v4.10-rc6

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Message-ID: <20170131221141.GA5379@htj.duckdns.org>
Date:   Tue, 31 Jan 2017 17:11:41 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dougmill@...ux.vnet.ibm.com
Subject: Re: [GIT PULL] percpu fix for v4.10-rc6

Hello, Linus.

On Tue, Jan 31, 2017 at 01:41:17PM -0800, Linus Torvalds wrote:
> On Tue, Jan 31, 2017 at 1:32 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I've pulled this, but I really think it's papering over the real
> > issue. Adding "linux-arch" mailing list to ask architecture
> > maintainers to check their implementation of the atomic ops that
> > return a truth value.

Yeah, for sure.

> For example, looking at the x86-32 version, I see this:
> 
>   static inline int atomic64_add_unless(atomic64_t *v, long long a, long long u)
>   ...
>         return (int)a;
> 
> which looks really horribly wrong, but the assembly implementation
> actually returns 0/1 in %eax so it ends up being right - just
> confusingly so.
> 
> Also, to make things more confusing, the underscore version
> (__atomic_add_unless()) actually returns the old value, not the truth
> value of the comparison.
> 
> So this area definitely is messy. The x86-64 versions actually look
> fairly clean and return nice boolean values.

We have a similar mess with bitops too.  x86 is cleaned up to have
bool returns but the generic implementation and a lot of other archs
return the tested bit instead of 1/0.  It'd be great to make all the
boolean functions actually return bool.

Thanks.

-- 
tejun