linux-kernel - Re: [ltt-dev] [PATCH] Fix dirty page accounting in redirty_page_for

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Message-ID: <20090430141050.GC14696@elte.hu>
Date:	Thu, 30 Apr 2009 16:10:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Tejun Heo <tj@...nel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Yuriy Lalym <ylalym@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ltt-dev@...ts.casi.polymtl.ca,
	Andrew Morton <akpm@...ux-foundation.org>, thomas.pi@...or.dea,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [ltt-dev] [PATCH] Fix dirty page accounting in
	redirty_page_for_writepage()


* Christoph Lameter <cl@...ux.com> wrote:

> On Thu, 30 Apr 2009, Ingo Molnar wrote:
> 
> > > I see however that it's only guaranteed to be atomic wrt preemption.
> >
> > That's really only true for the non-x86 fallback defines. If we so
> > decide, we could make the fallbacks in asm-generic/percpu.h irq-safe
> 
> The fallbacks have different semantics and therefore we cannot 
> rely on irq safeness in the core code when using the x86 cpu ops.

Well it's irq and preempt safe on x86.

It's preempt-safe on other architectures - but the fallback is not 
irq-safe on other architectures. That is remedied easily via the 
patch below. (Note: totally untested)

	Ingo

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 1581ff2..6b3984a 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -139,17 +139,23 @@ static inline void free_percpu(void *p)
 #ifndef percpu_read
 # define percpu_read(var)						\
   ({									\
+	unsigned long flags;						\
 	typeof(per_cpu_var(var)) __tmp_var__;				\
-	__tmp_var__ = get_cpu_var(var);					\
-	put_cpu_var(var);						\
+									\
+	local_irq_save(flags);						\
+	__tmp_var__ = __get_cpu_var(var);				\
+	local_irq_restore(flags);					\
 	__tmp_var__;							\
   })
 #endif
 
 #define __percpu_generic_to_op(var, val, op)				\
 do {									\
-	get_cpu_var(var) op val;					\
-	put_cpu_var(var);						\
+	unsigned long flags;						\
+									\
+	local_irq_save(flags);						\
+	op val;								\
+	local_irq_restore(flags);					\
 } while (0)
 
 #ifndef percpu_write
--
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