linux-stable/include/linux/preempt.h
Nicolas Pitre d6f029130f [PATCH] fix race with preempt_enable()
Currently a simple

	void foo(void) { preempt_enable(); }

produces the following code on ARM:

foo:
	bic	r3, sp, #8128
	bic	r3, r3, #63
	ldr	r2, [r3, #4]
	ldr	r1, [r3, #0]
	sub	r2, r2, #1
	tst	r1, #4
	str	r2, [r3, #4]
	blne	preempt_schedule
	mov	pc, lr

The problem is that the TIF_NEED_RESCHED flag is loaded _before_ the
preemption count is stored back, hence any interrupt coming within that
3 instruction window causing TIF_NEED_RESCHED to be set won't be
seen and scheduling won't happen as it should.

Nothing currently prevents gcc from performing that reordering.  There
is already a barrier() before the decrement of the preemption count, but
another one is needed between this and the TIF_NEED_RESCHED flag test
for proper code ordering.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2005-12-22 09:17:39 -08:00

64 lines
1.4 KiB
C

#ifndef __LINUX_PREEMPT_H
#define __LINUX_PREEMPT_H
/*
* include/linux/preempt.h - macros for accessing and manipulating
* preempt_count (used for kernel preemption, interrupt count, etc.)
*/
#include <linux/config.h>
#include <linux/thread_info.h>
#include <linux/linkage.h>
#ifdef CONFIG_DEBUG_PREEMPT
extern void fastcall add_preempt_count(int val);
extern void fastcall sub_preempt_count(int val);
#else
# define add_preempt_count(val) do { preempt_count() += (val); } while (0)
# define sub_preempt_count(val) do { preempt_count() -= (val); } while (0)
#endif
#define inc_preempt_count() add_preempt_count(1)
#define dec_preempt_count() sub_preempt_count(1)
#define preempt_count() (current_thread_info()->preempt_count)
#ifdef CONFIG_PREEMPT
asmlinkage void preempt_schedule(void);
#define preempt_disable() \
do { \
inc_preempt_count(); \
barrier(); \
} while (0)
#define preempt_enable_no_resched() \
do { \
barrier(); \
dec_preempt_count(); \
} while (0)
#define preempt_check_resched() \
do { \
if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
preempt_schedule(); \
} while (0)
#define preempt_enable() \
do { \
preempt_enable_no_resched(); \
barrier(); \
preempt_check_resched(); \
} while (0)
#else
#define preempt_disable() do { } while (0)
#define preempt_enable_no_resched() do { } while (0)
#define preempt_enable() do { } while (0)
#define preempt_check_resched() do { } while (0)
#endif
#endif /* __LINUX_PREEMPT_H */