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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Kees Cook <keescook@chromium.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: Fri, 14 Jun 2019 12:24:54 +0200
Message-ID: <CAKv+Gu_Kdq=UPijjA84FpmO=ZsdEO9EyyF7GeOQ+WmfqtO_hMg@mail.gmail.com> (raw)
In-Reply-To: <20190614095846.GC10506@fuggles.cambridge.arm.com>

On Fri, 14 Jun 2019 at 11:58, Will Deacon <will.deacon@arm.com> wrote:
>
> [+Kees]
>
> On Fri, Jun 14, 2019 at 07:09:26AM +0000, Jayachandran Chandrasekharan Nair wrote:
> > On Wed, Jun 12, 2019 at 10:31:53AM +0100, Will Deacon wrote:
> > > On Wed, Jun 12, 2019 at 04:10:20AM +0000, Jayachandran Chandrasekharan Nair wrote:
> > > > Now that the lockref change is mainline, I think we need to take another
> > > > look at this patch.
> > >
> > > Before we get too involved with this, I really don't want to start a trend of
> > > "let's try to rewrite all code using cmpxchg() in Linux because of TX2".
> >
> > x86 added a arch-specific fast refcount implementation - and the commit
> > specifically notes that it is faster than cmpxchg based code[1].
> >
> > There seems to be an ongoing effort to move over more and more subsystems
> > from atomic_t to refcount_t(e.g.[2]), specifically because refcount_t on
> > x86 is fast enough and you get some error checking atomic_t that does not
> > have.
>
> Correct, but there are also some cases that are only caught by
> REFCOUNT_FULL.
>

Yes, but do note that my arm64 implementation catches
increment-from-zero as well.

> > > At some point, the hardware needs to play ball. However...
> >
> > Even on a totally baller CPU, REFCOUNT_FULL is going to be slow :)
> > On TX2, this specific benchmark just highlights the issue, but the
> > difference is significant even on x86 (as noted above).
>
> My point was more general than that. If you want scalable concurrent code,
> then you end up having to move away from the serialisation introduced by
> locking. The main trick in the toolbox is cmpxchg() so, in the absence of
> a zoo of special-purpose atomic instructions, it really needs to do better
> than serialising.
>
> > > I was hoping we could use LDMIN/LDMAX to maintain the semantics of
> > > REFCOUNT_FULL, but now that I think about it I can't see how we could keep
> > > the arithmetic atomic in that case. Hmm.
> >
> > Do you think Ard's patch needs changes before it can be considered? I
> > can take a look at that.
>
> I would like to see how it performs if we keep the checking inline, yes.
> I suspect Ard could spin this in short order.
>

Moving the post checks before the stores you mean? That shouldn't be
too difficult, I suppose, but it will certainly cost performance.

> > > Whatever we do, I prefer to keep REFCOUNT_FULL the default option for arm64,
> > > so if we can't keep the semantics when we remove the cmpxchg, you'll need to
> > > opt into this at config time.
> >
> > Only arm64 and arm selects REFCOUNT_FULL in the default config. So please
> > reconsider this! This is going to slow down arm64 vs. other archs and it
> > will become worse when more code adopts refcount_t.
>
> Maybe, but faced with the choice between your micro-benchmark results and
> security-by-default for people using the arm64 Linux kernel, I really think
> that's a no-brainer. I'm well aware that not everybody agrees with me on
> that.

I think the question whether the benchmark is valid is justified, but
otoh, we are obsessed with hackbench which is not that representative
of a real workload either. It would be better to discuss these changes
in the context of known real-world use cases where refcounts are a
true bottleneck.

Also, I'd like to have Kees's view on the gap between REFCOUNT_FULL
and the fast version on arm64. I'm not convinced the cases we are not
covering are such a big deal.

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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 [this message]
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
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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