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From: Kees Cook <keescook@chromium.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	Jan Glauber <jglauber@marvell.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC] Disable lockref on arm64
Date: Sat, 15 Jun 2019 06:59:33 -0700
Message-ID: <201906150654.FF4400F7C8@keescook> (raw)
In-Reply-To: <CAKv+Gu_XuhgUCYOeykrbaxJz-wL1HFrc_O+HeZHqaGkMHd2J9Q@mail.gmail.com>

On Sat, Jun 15, 2019 at 10:47:19AM +0200, Ard Biesheuvel wrote:
> remaining question Will had was whether it makes sense to do the
> condition checks before doing the actual store, to avoid having a time
> window where the refcount assumes its illegal value. Since arm64 does
> not have memory operands, the instruction count wouldn't change, but
> it will definitely result in a performance hit on out-of-order CPUs.

What do the races end up looking like? Is it possible to have two
threads ordered in a way that a second thread could _un_saturate a
counter?

CPU 1                   CPU 2
inc()
  load INT_MAX-1
  about to overflow?
  yes
                        dec()
                          load INT_MAX-1
  set to INT_MAX
                          set to INT_MAX-2

Or would you use the same INT_MIN/2 saturation point done on x86?

As for performance, it should be easy to measure with the LKDTM test
to find out exactly the differences.

-- 
Kees Cook

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Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 14:52 Jan Glauber
2019-05-01 16:01 ` Will Deacon
2019-05-02  8:38   ` Jan Glauber
2019-05-01 16:41 ` Linus Torvalds
2019-05-02  8:27   ` Jan Glauber
2019-05-02 16:12     ` Linus Torvalds
2019-05-02 23:19       ` Jayachandran Chandrasekharan Nair
2019-05-03 19:40         ` Linus Torvalds
2019-05-06  6:13           ` [EXT] " Jayachandran Chandrasekharan Nair
2019-05-06 17:13             ` Linus Torvalds
2019-05-06 18:10             ` Will Deacon
2019-05-18  4:24               ` Jayachandran Chandrasekharan Nair
2019-05-18 10:00                 ` Ard Biesheuvel
2019-05-22 16:04                   ` Will Deacon
2019-06-12  4:10                     ` Jayachandran Chandrasekharan Nair
2019-06-12  9:31                       ` Will Deacon
2019-06-14  7:09                         ` Jayachandran Chandrasekharan Nair
2019-06-14  9:58                           ` Will Deacon
2019-06-14 10:24                             ` Ard Biesheuvel
2019-06-14 10:38                               ` Will Deacon
2019-06-15  4:21                                 ` Kees Cook
2019-06-15  8:47                                   ` Ard Biesheuvel
2019-06-15 13:59                                     ` Kees Cook [this message]
2019-06-15 14:18                                       ` Ard Biesheuvel
2019-06-16 21:31                                         ` Kees Cook
2019-06-17 11:33                                           ` Ard Biesheuvel
2019-06-17 17:26                                             ` Will Deacon
2019-06-17 20:07                                               ` Jayachandran Chandrasekharan Nair
2019-06-18  5:41                                               ` Kees Cook
2019-06-13  9:53                       ` Hanjun Guo
2019-06-05 13:48   ` [PATCH] lockref: Limit number of cmpxchg loop retries Jan Glauber
2019-06-05 20:16     ` Linus Torvalds
2019-06-06  8:03       ` Jan Glauber
2019-06-06  9:41         ` Will Deacon
2019-06-06 10:28           ` Jan Glauber
2019-06-07  7:27             ` Jan Glauber
2019-06-07 20:14               ` Linus Torvalds

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