Commit d6f02913 authored by Nicolas Pitre's avatar Nicolas Pitre Committed by Linus Torvalds
Browse files

[PATCH] fix race with preempt_enable()

Currently a simple

	void foo(void) { preempt_enable(); }

produces the following code on ARM:

	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: default avatarNicolas Pitre <>
Acked-by: default avatarNick Piggin <>
Signed-off-by: default avatarLinus Torvalds <>
parent d5ea4e26
......@@ -48,6 +48,7 @@ do { \
#define preempt_enable() \
do { \
preempt_enable_no_resched(); \
barrier(); \
preempt_check_resched(); \
} while (0)
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