From: Nicolas Pitre <nico@cam.org>
Date: Tue, 20 Nov 2007 16:20:29 +0000 (+0100)
Subject: [ARM] 4659/1: remove possibilities for spurious false negative with __kuser_cmpxchg
X-Git-Tag: v2.6.24-rc4~19^2~3
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b49c0f24cf6744a3f4fd09289fe7cade349dead5;p=pandora-kernel.git

[ARM] 4659/1: remove possibilities for spurious false negative with __kuser_cmpxchg

The ARM __kuser_cmpxchg routine is meant to implement an atomic cmpxchg
in user space.  It however can produce spurious false negative if a
processor exception occurs in the middle of the operation.  Normally
this is not a problem since cmpxchg is typically called in a loop until
it succeeds to implement an atomic increment for example.

Some use cases which don't involve a loop require that the operation be
100% reliable though.  This patch changes the implementation so to
reattempt the operation after an exception has occurred in the critical
section rather than abort it.

Here's a simple program to test the fix (don't use CONFIG_NO_HZ in your
kernel as this depends on a sufficiently high interrupt rate):

	#include <stdio.h>

	typedef int (__kernel_cmpxchg_t)(int oldval, int newval, int *ptr);
	#define __kernel_cmpxchg (*(__kernel_cmpxchg_t *)0xffff0fc0)

	int main()
	{
		int i, x = 0;
		for (i = 0; i < 100000000; i++) {
			int v = x;
			if (__kernel_cmpxchg(v, v+1, &x))
				printf("failed at %d: %d vs %d\n", i, v, x);
		}
		printf("done with %d vs %d\n", i, x);
		return 0;
	}

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
---

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