[PATCH] Conditionally check expected_preempt_count in __resched_legal()

Commit 15f8bec6987d0b05e2f07d8b31471077f49df8c1 ("fix cond_resched() fix")
introduced an 'expected_preempt_count' parameter to __resched_legal() to
fix a bug where it was returning a false negative when called from
cond_resched_lock() and preemption was enabled.

Unfortunately this broke things for when preemption is disabled.
preempt_count() will always return zero, thus failing the check against any
value of expected_preempt_count not equal to zero.  cond_resched_lock() for
example, passes an expected_preempt_count value of 1.

So fix the fix for the cond_resched() fix by skipping the check of
preempt_count() against expected_preempt_count when preemption is disabled.

Credit should go to Sunil Mushran for spotting the bug during testing.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/kernel/sched.c b/kernel/sched.c
index b81ed8703f70a71a680bd4652cb82818d939662b..850bde4b31a210a38b8443e4e4dc2ba345a04437 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4619,8 +4619,10 @@ asmlinkage long sys_sched_yield(void)
 
 static inline int __resched_legal(int expected_preempt_count)
 {
+#ifdef CONFIG_PREEMPT
        if (unlikely(preempt_count() != expected_preempt_count))
                return 0;
+#endif
        if (unlikely(system_state != SYSTEM_RUNNING))
                return 0;
        return 1;