From: Kees Cook <keescook@chromium.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
Jan Glauber <jglauber@marvell.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: Mon, 17 Jun 2019 22:41:41 -0700 [thread overview]
Message-ID: <201906172233.F753B92@keescook> (raw)
In-Reply-To: <20190617172620.GK30800@fuggles.cambridge.arm.com>
On Mon, Jun 17, 2019 at 06:26:20PM +0100, Will Deacon wrote:
> On Mon, Jun 17, 2019 at 01:33:19PM +0200, Ard Biesheuvel wrote:
> > On my single core TX2, the comparative performance is as follows
> >
> > Baseline: REFCOUNT_TIMING test using REFCOUNT_FULL (LSE cmpxchg)
> > 191057942484 cycles # 2.207 GHz
> > 148447589402 instructions # 0.78 insn per
> > cycle
> >
> > 86.568269904 seconds time elapsed
> >
> > Upper bound: ATOMIC_TIMING
> > 116252672661 cycles # 2.207 GHz
> > 28089216452 instructions # 0.24 insn per
> > cycle
> >
> > 52.689793525 seconds time elapsed
> >
> > REFCOUNT_TIMING test using LSE atomics
> > 127060259162 cycles # 2.207 GHz
>
> Ok, so assuming JC's complaint is valid, then these numbers are compelling.
> In particular, my understanding of this thread is that your optimised
> implementation doesn't actually sacrifice any precision; it just changes
> the saturation behaviour in a way that has no material impact. Kees, is that
> right?
That is my understanding, yes. There is no loss to detection precision.
But for clarity, I should point out it has one behavioral change that is
the same change as on x86: the counter is now effectively a 31 bit counter
not a 32 bit counter, as the signed bit is being used for saturation.
> If so, I'm not against having this for arm64, with the premise that we can
> hide the REFCOUNT_FULL option entirely given that it would only serve to
> confuse if exposed.
If the LSE atomics version has overflow, dec-to-zero, and inc-from-zero
protections, then as far as I'm concerned, REFCOUNT_FULL doesn't need
to exist for arm64. On the Kconfig front, as long as there isn't a way
to revert refcount_t to atomic_t, I'm happy. :)
--
Kees Cook
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next prev parent reply other threads:[~2019-06-18 5:41 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-29 14:52 [RFC] Disable lockref on arm64 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
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 [this message]
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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