From: Andrew Morton <akpm@linux-foundation.org>
To: Huang Ying <ying.huang@intel.com>
Cc: Len Brown <lenb@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Andi Kleen <andi@firstfloor.org>,
"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
Mauro Carvalho Chehab <mchehab@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH -v4 1/2] lib, Make gen_pool memory allocator lockless
Date: Tue, 16 Nov 2010 19:57:20 -0800 [thread overview]
Message-ID: <20101116195720.23287038.akpm@linux-foundation.org> (raw)
In-Reply-To: <1289963005.8719.1238.camel@yhuang-dev>
On Wed, 17 Nov 2010 11:03:25 +0800 Huang Ying <ying.huang@intel.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.
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.
> 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.
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).
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.
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..." ;)
next prev parent reply other threads:[~2010-11-17 4:01 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-11-16 0:53 [PATCH -v4 0/2] Lockless memory allocator and list Huang Ying
2010-11-16 0:53 ` [PATCH -v4 1/2] lib, Make gen_pool memory allocator lockless Huang Ying
2010-11-16 21:50 ` Andrew Morton
2010-11-17 2:18 ` Huang Ying
2010-11-17 2:35 ` Andrew Morton
2010-11-17 3:03 ` Huang Ying
2010-11-17 3:57 ` Andrew Morton [this message]
2010-11-17 6:05 ` Huang Ying
2010-11-17 10:49 ` Peter Zijlstra
2010-11-17 11:16 ` huang ying
2010-11-17 11:38 ` Peter Zijlstra
2010-11-17 10:40 ` Peter Zijlstra
2010-11-17 11:47 ` huang ying
2010-11-17 11:53 ` Peter Zijlstra
2010-11-18 1:14 ` Huang Ying
2010-11-18 8:34 ` Peter Zijlstra
2010-11-18 8:43 ` Paul Mundt
2010-11-18 8:57 ` Peter Zijlstra
2010-11-18 9:03 ` Paul Mundt
2010-11-16 0:53 ` [PATCH -v4 2/2] lib, Add lock-less NULL terminated single list Huang Ying
2010-11-16 11:50 ` Peter Zijlstra
2010-11-16 16:33 ` Linus Torvalds
2010-11-16 11:49 ` [PATCH -v4 0/2] Lockless memory allocator and list Peter Zijlstra
2010-11-16 16:38 ` Linus Torvalds
2010-11-16 18:04 ` Peter Zijlstra
2010-11-17 1:45 ` Huang Ying
2010-11-17 1:03 ` Huang Ying
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