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

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Message-ID: <20180215182049.GC15274@arm.com>
Date:   Thu, 15 Feb 2018 18:20:49 +0000
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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_*

Hi Peter,

Thanks for having a look.

On Thu, Feb 15, 2018 at 06:08:47PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 15, 2018 at 03:29:33PM +0000, Will Deacon wrote:
> > +static inline void __clear_bit_unlock(unsigned int nr,
> > +				      volatile unsigned long *p)
> > +{
> > +	unsigned long old;
> >  
> > +	p += BIT_WORD(nr);
> > +	old = READ_ONCE(*p);
> > +	old &= ~BIT_MASK(nr);
> > +	smp_store_release(p, old);
> 
> This should be atomic_set_release() I think, for the special case where
> atomics are implemented with spinlocks, see the 'fun' case in
> Documentation/atomic_t.txt.

My understanding of __clear_bit_unlock is that there is guaranteed to be
no concurrent accesses to the same word, so why would it matter whether
locks are used to implement atomics?

> 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...

> 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!

Will