linux-kernel - Re: [PATCH 1/4] bitops: Introduce assign

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Date:   Tue, 22 Aug 2017 12:04:47 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Bart Van Assche <Bart.VanAssche@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Neil Brown <neilb@...e.com>, Ingo Molnar <mingo@...hat.com>,
        Theodore Ts'o <tytso@....edu>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "agk@...hat.com" <agk@...hat.com>,
        "phil@...pberrypi.org" <phil@...pberrypi.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "m.duckeck@...bus.de" <m.duckeck@...bus.de>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] bitops: Introduce assign_bit()

On Tue, Aug 22, 2017 at 11:27:31AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 22, 2017 at 10:30:50AM +0200, Lukas Wunner wrote:
> > diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> > index a83c822c35c2..097af36887c0 100644
> > --- a/include/linux/bitops.h
> > +++ b/include/linux/bitops.h
> > @@ -226,6 +226,30 @@ static inline unsigned long __ffs64(u64 word)
> >  	return __ffs((unsigned long)word);
> >  }
> >  
> > +/**
> > + * assign_bit - Assign value to a bit in memory
> > + * @value: the value to assign
> > + * @nr: the bit to set
> > + * @addr: the address to start counting from
> > + */
> > +static __always_inline void assign_bit(bool value, long nr,
> > +				       volatile unsigned long *addr)
> > +{
> > +	if (value)
> > +		set_bit(nr, addr);
> > +	else
> > +		clear_bit(nr, addr);
> > +}
> > +
> > +static __always_inline void __assign_bit(bool value, long nr,
> > +					 volatile unsigned long *addr)
> > +{
> > +	if (value)
> > +		__set_bit(nr, addr);
> > +	else
> > +		__clear_bit(nr, addr);
> > +}
> > +
> 
> I dislike the argument order, in C you naturally write: dst = src. So I
> would have expected:
> 
> 	assign_bit(nr, addr, val);
> 
> but we have quite a few of these backwards functions in the kernel (like
> most of the atomic_t family) and I didn't check to see if the existing
> bitops are part of that 'tradition'.

The functions in include/linux/bitmap.h do follow the dst-then-src
pattern.  I carried over the argument order from Bart's function
to minimize the impact on the md subsystem, but will be happy to
respin with the order you're suggesting.  Will wait a bit though
to see if there are further comments.

Thanks,

Lukas