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 <[email protected]>
---
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index d9a2f52..5769d14 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -48,6 +48,7 @@ do { \
#define preempt_enable() \
do { \
preempt_enable_no_resched(); \
+ barrier(); \
preempt_check_resched(); \
} while (0)
-
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