linux-kernel - Re: [RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using atomic_fetch

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Message-ID: <20180216103520.GC25201@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Feb 2018 11:35:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mingo@...nel.org
Subject: Re: [RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using
 atomic_fetch_*

On Thu, Feb 15, 2018 at 06:20:49PM +0000, Will Deacon wrote:

> > The only other comment is that I think it would be better if you use
> > atomic_t instead of atomic_long_t. It would just mean changing
> > BIT_WORD() and BIT_MASK().
> 
> It would make it pretty messy for big-endian architectures, I think...

Urgh, the big.little indians strike again.. Bah I always forget about
that.

#define BIT_U32_MASK(nr)	(1UL << ((nr) % 32))
#define BIT_U32_WORD(nr)	(((nr) / 32) ^ (4 * __BIG_ENDIAN__))

Or something like that might work, but I always get these things wrong.

> > The reason is that we generate a pretty sane set of atomic_t primitives
> > as long as the architecture supplies cmpxchg, but atomic64 defaults to
> > utter crap, even on 64bit platforms.
> 
> I think all the architectures using this today are 32-bit:
> 
> blackfin
> c6x
> cris
> metag
> openrisc
> sh
> xtensa
> 
> and I don't know how much we should care about optimising the generic atomic
> bitops for 64-bit architectures that rely on spinlocks for 64-bit atomics!

You're probably right, but it just bugs me that we default to such
horrible crap. Arguably we should do a better default for atomic64_t on
64bit archs. But that's for another time.