linux-kernel - Re: [RFC][PATCH 5/5] arch: Sanitize atomic

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Date:	Thu, 6 Feb 2014 18:52:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Will Deacon <will.deacon@....com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 5/5] arch: Sanitize atomic_t bitwise ops

On Thu, Feb 06, 2014 at 08:53:02AM -0800, Linus Torvalds wrote:
> On Thu, Feb 6, 2014 at 5:48 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > Many archs have atomic_{set,clear}_mask() but not all. Remove these
> > and provide a comprehensive set of bitops:
> >
> >   atomic{,64}_{and,or,xor}{,_return}()
> 
> Who uses these, and why?

s390 is stuffed with atomic_{set,clear}_mask usage, no clue whatfor.

There's 2 atomic_set_mask() users in drm/i915, again haven't looked if
it makes sense.

Various archs use atomic_set_mask() for their tlb flush mask, which
arguably should be done using the bitmap functions we have.

> The "_return()" versions of atomic ops are noticeably slower and more
> complex on common architectures (ie x86), and apparently there is no
> use of them since they didn't exist.
> 
> So why add them? Just to encourage people to do bad things?

Fair enough, they're mostly an accident of how I implemented the macro
generation magic. I suppose I can change that and avoid generating the
_return thingies.

I don't particularly care for this patch too much; but I do dislike the
atomic_{set,clear}_mask() things -- mostly because they don't really fit
the normal atomic_t functions and because some archs have then and
others do no.

And atomic_{set,clear}_mask() can be used for setting/clearing multiple
bits as opposed to {set,clear}_bit, which only does a single bit.

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