From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id F2B578E0002 for ; Mon, 14 Jan 2019 14:00:34 -0500 (EST) Received: by mail-ed1-f70.google.com with SMTP id z10so189583edz.15 for ; Mon, 14 Jan 2019 11:00:34 -0800 (PST) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id i3si4020379edk.411.2019.01.14.11.00.33 for ; Mon, 14 Jan 2019 11:00:33 -0800 (PST) Date: Mon, 14 Jan 2019 19:00:25 +0000 From: Will Deacon Subject: Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value Message-ID: <20190114190025.GA29167@fuggles.cambridge.arm.com> References: <1547166387-19785-1-git-send-email-vgupta@synopsys.com> <1547166387-19785-4-git-send-email-vgupta@synopsys.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1547166387-19785-4-git-send-email-vgupta@synopsys.com> Sender: owner-linux-mm@kvack.org List-ID: To: Vineet Gupta Cc: linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-mm@kvack.org, peterz@infradead.org, Miklos Szeredi , Ingo Molnar , Jani Nikula , Chris Wilson , Andrew Morton 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 > Cc: Miklos Szeredi > Cc: Ingo Molnar > Cc: Jani Nikula > Cc: Chris Wilson > Cc: Andrew Morton > Cc: Will Deacon > Signed-off-by: Vineet Gupta > --- > 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 May also explain why no in-tree users appear to use the return value! Will