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Subject[patch 59/59] sched: fix cond_resched_softirq() offset
-stable review patch.  If anyone has any objections, please let us know.
------------------
From: Ingo Molnar <mingo@elte.hu>

Remove the __resched_legal() check: it is conceptually broken. The biggest
problem it had is that it can mask buggy cond_resched() calls. A
cond_resched() call is only legal if we are not in an atomic context, with
two narrow exceptions:

- if the system is booting
- a reacquire_kernel_lock() down() done while PREEMPT_ACTIVE is set

But __resched_legal() hid this and just silently returned whenever
these primitives were called from invalid contexts. (Same goes for
cond_resched_locked() and cond_resched_softirq()).

Furthermore, the __legal_resched(0) call was buggy in that it caused
unnecessarily long softirq latencies via cond_resched_softirq(). (which is
only called from softirq-off sections, hence the code did nothing.)

The fix is to resurrect the efficiency of the might_sleep checks and to
only allow the narrow exceptions.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
[chrisw: backport to 2.6.19.2]
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---
kernel/sched.c | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)
--- linux-2.6.19.2.orig/kernel/sched.c
+++ linux-2.6.19.2/kernel/sched.c
@@ -4524,15 +4524,6 @@ asmlinkage long sys_sched_yield(void)
return 0;
}

-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)
{
#ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
@@ -4552,7 +4543,8 @@ static void __cond_resched(void)

int __sched cond_resched(void)
{
- if (need_resched() && __resched_legal(0)) {
+ if (need_resched() && !(preempt_count() & PREEMPT_ACTIVE) &&
+ system_state == SYSTEM_RUNNING) {
__cond_resched();
return 1;
}
@@ -4578,7 +4570,7 @@ int cond_resched_lock(spinlock_t *lock)
ret = 1;
spin_lock(lock);
}
- if (need_resched() && __resched_legal(1)) {
+ if (need_resched() && system_state == SYSTEM_RUNNING) {
spin_release(&lock->dep_map, 1, _THIS_IP_);
_raw_spin_unlock(lock);
preempt_enable_no_resched();
@@ -4594,7 +4586,7 @@ int __sched cond_resched_softirq(void)
{
BUG_ON(!in_softirq());

- if (need_resched() && __resched_legal(0)) {
+ if (need_resched() && system_state == SYSTEM_RUNNING) {
raw_local_irq_disable();
_local_bh_enable();
raw_local_irq_enable();
--
-
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