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

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Message-ID: <CA+55aFx=dDhYzp7yN5oiPT+_Gyffo+0hT5WRFd7eGeMz8oNnDg@mail.gmail.com>
Date:	Thu, 6 Feb 2014 09:56:54 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.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 6, 2014 at 9:52 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> s390 is stuffed with atomic_{set,clear}_mask usage, no clue whatfor.

It's not the set/clear_mask ones I would worry about.

It's the "_return" variants. As far as I can tell, there are exactly
ZERO users of that stuff, and they are BAD BAD BAD.

On x86, those things would cause a cmpxchg loop, and the bad
read-for-shared-before-acquire cacheline pattern.

So why indirectly encourage people to add users for a bad operation?

                        Linus
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