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Subject[patch 18/23] cond_resched() fix

-stable review patch. If anyone has any objections, please let us know.

------------------
From: Andrew Morton <akpm@osdl.org>
[PATCH] cond_resched() fix

Fix a bug identified by Zou Nan hai <nanhai.zou@intel.com>:

If the system is in state SYSTEM_BOOTING, and need_resched() is true,
cond_resched() returns true even though it didn't reschedule. Consequently
need_resched() remains true and JBD locks up.

Fix that by teaching cond_resched() to only return true if it really did call
schedule().

cond_resched_lock() and cond_resched_softirq() have a problem too. If we're
in SYSTEM_BOOTING state and need_resched() is true, these functions will drop
the lock and will then try to call schedule(), but the SYSTEM_BOOTING state
will prevent schedule() from being called. So on return, need_resched() will
still be true, but cond_resched_lock() has to return 1 to tell the caller that
the lock was dropped. The caller will probably lock up.

Bottom line: if these functions dropped the lock, they _must_ call schedule()
to clear need_resched(). Make it so.

Also, uninline __cond_resched(). It's largeish, and slowpath.

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/sched.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)
--- linux-2.6.17.7.orig/kernel/sched.c
+++ linux-2.6.17.7/kernel/sched.c
@@ -4044,17 +4044,22 @@ asmlinkage long sys_sched_yield(void)
return 0;
}

-static inline void __cond_resched(void)
+static inline int __resched_legal(int expected_preempt_count)
+{
+ if (unlikely(preempt_count() != expected_preempt_count))
+ return 0;
+ if (unlikely(system_state != SYSTEM_RUNNING))
+ return 0;
+ return 1;
+}
+
+static void __cond_resched(void)
{
/*
* The BKS might be reacquired before we have dropped
* PREEMPT_ACTIVE, which could trigger a second
* cond_resched() call.
*/
- if (unlikely(preempt_count()))
- return;
- if (unlikely(system_state != SYSTEM_RUNNING))
- return;
do {
add_preempt_count(PREEMPT_ACTIVE);
schedule();
@@ -4064,13 +4069,12 @@ static inline void __cond_resched(void)

int __sched cond_resched(void)
{
- if (need_resched()) {
+ if (need_resched() && __resched_legal(0)) {
__cond_resched();
return 1;
}
return 0;
}
-
EXPORT_SYMBOL(cond_resched);

/*
@@ -4091,7 +4095,7 @@ int cond_resched_lock(spinlock_t *lock)
ret = 1;
spin_lock(lock);
}
- if (need_resched()) {
+ if (need_resched() && __resched_legal(1)) {
_raw_spin_unlock(lock);
preempt_enable_no_resched();
__cond_resched();
@@ -4100,14 +4104,13 @@ int cond_resched_lock(spinlock_t *lock)
}
return ret;
}
-
EXPORT_SYMBOL(cond_resched_lock);

int __sched cond_resched_softirq(void)
{
BUG_ON(!in_softirq());

- if (need_resched()) {
+ if (need_resched() && __resched_legal(0)) {
__local_bh_enable();
__cond_resched();
local_bh_disable();
@@ -4115,10 +4118,8 @@ int __sched cond_resched_softirq(void)
}
return 0;
}
-
EXPORT_SYMBOL(cond_resched_softirq);

-
/**
* yield - yield the current processor to other threads.
*
--
-
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