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Message-ID: <1289973914.8719.1261.camel@yhuang-dev>
Date: Wed, 17 Nov 2010 14:05:14 +0800
From: Huang Ying <ying.huang@...el.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Len Brown <lenb@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Andi Kleen <andi@...stfloor.org>,
"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...e.hu>,
Mauro Carvalho Chehab <mchehab@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH -v4 1/2] lib, Make gen_pool memory allocator lockless
On Wed, 2010-11-17 at 11:57 +0800, Andrew Morton wrote:
> On Wed, 17 Nov 2010 11:03:25 +0800 Huang Ying <ying.huang@...el.com> wrote:
>
> > It seems that Steven thinks many architectures without NMI-safe cmpxchg
> > have no real NMI too.
>
> Could be.
>
> Really, we should nail this down and work out the matrix of
> what-works-with-what, and then arrange for the things which _won't_
> work to be either non-Kconfigurable or non-compilable.
Yes. I will try to work out a draft matrix for review firstly.
> The worst thing we could do would be to advertise some code as
> "nmi-safe!!", then to have someone go and use it on that basis, only
> for their users to later discover ghastly rare races.
Yes.
> > In the patch description and comments, it is said that on architectures
> > without NMI-safe cmpxchg, gen_pool can not be used in NMI handler
> > safely.
> >
> > Or do you think it is better to use a spin_trylock based fallback if
> > NMI-safe cmpxchg is not available? Or require cmpxchg implementation
> > uses spin_trylock instead of spin_lock?
>
> As a first step, a typical thing to do would be to create
> CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG, define that in the appropriate
> architectures, make ftrace and perf and genpool and anything else
> dependent upon that at Kconfig-time.
Yes. At least lockless list should depend on that. And we need select
between cmpxchg and spin_trylock_irqsave based on that.
Hi, Peter,
Do you think think irq_work should depend on that? Or we just
reimplement irq_work based on lockless list and make irq_work depends on
lockless list?
> A spin_trylock_irqsave() implementation would do what? Rarely fail the
> memory allocation attempt if the trylock failed? I guess that's
> acceptable in the context of gen_pool, because memory allocators are
> expected to fail, and everyone carefully tests the allocation-failed
> error paths (lol).
Yes. I will implement the fallback for gen_pool.
> But rare failures may not be useful within the context of future
> clients of the "lockless" list implementation so I'd say that a safer
> approach would be to make the list implementation require
> CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG and be done with it.
Yes.
> So that's all pretty simple so far. However...
>
> The list implementation is still useful to
> non-CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG clients, as long as they aren't
> using it from NMI context, yes?
>
> In which case I suppose we could add a library of lockless_list_foo()
> functions which are usable from non-NMI contexts and which are
> available to all configs. And on top of that implement a library of
> nmi_safe_lockless_list_foo() functions which are just wrappers around
> lockless_list_foo(), but which are only available if
> CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG=y.
>
> Which is getting to be a bit of a pain, so you might choose to
> disregard everything after "However..." ;)
At least as the first step, I prefer to just make lockless list depend
on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
Best Regards,
Huang Ying
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