[PATCH] sched: fix cond_resched_softirq() offset

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>

diff --git a/kernel/sched.c b/kernel/sched.c
index 5f6939cad31476fd690c63d44ca4762ad33b9de8..ee200b2ac8dc1bee144242a0cb1ae6b199c53113 100644 (file)
--- a/kernel/sched.c
+++ b/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();