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Message-ID: <20190114190025.GA29167@fuggles.cambridge.arm.com> Date: Mon, 14 Jan 2019 19:00:25 +0000 From: Will Deacon <will.deacon@....com> To: Vineet Gupta <vineet.gupta1@...opsys.com> Cc: linux-kernel@...r.kernel.org, linux-snps-arc@...ts.infradead.org, linux-mm@...ck.org, peterz@...radead.org, Miklos Szeredi <mszeredi@...hat.com>, Ingo Molnar <mingo@...nel.org>, Jani Nikula <jani.nikula@...el.com>, Chris Wilson <chris@...is-wilson.co.uk>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value On Thu, Jan 10, 2019 at 04:26:27PM -0800, Vineet Gupta wrote: > | > Also, set_mask_bits is used in fs quite a bit and we can possibly come up > | > with a generic llsc based implementation (w/o the cmpxchg loop) > | > | May I also suggest changing the return value of set_mask_bits() to old. > | > | You can compute the new value given old, but you cannot compute the old > | value given new, therefore old is the better return value. Also, no > | current user seems to use the return value, so changing it is without > | risk. > > Link: http://lkml.kernel.org/g/20150807110955.GH16853@twins.programming.kicks-ass.net > Suggested-by: Peter Zijlstra <peterz@...radead.org> > Cc: Miklos Szeredi <mszeredi@...hat.com> > Cc: Ingo Molnar <mingo@...nel.org> > Cc: Jani Nikula <jani.nikula@...el.com> > Cc: Chris Wilson <chris@...is-wilson.co.uk> > Cc: Andrew Morton <akpm@...ux-foundation.org> > Cc: Will Deacon <will.deacon@....com> > Signed-off-by: Vineet Gupta <vgupta@...opsys.com> > --- > include/linux/bitops.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 705f7c442691..602af23b98c7 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -246,7 +246,7 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, > new__ = (old__ & ~mask__) | bits__; \ > } while (cmpxchg(ptr, old__, new__) != old__); \ > \ > - new__; \ > + old__; \ > }) > #endif Acked-by: Will Deacon <will.deacon@....com> May also explain why no in-tree users appear to use the return value! Will
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