Re: Linux 2.6.20-rc2

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Hi.
Can you confirm that the problem I mentioned in
http://lkml.org/lkml/2006/12/24/32 is the same?

Best regards,
Fabio




On 12/26/06, Ingo Molnar <[email protected]> wrote:

* Ingo Molnar <[email protected]> wrote:

> > I've had at least one more occurrence of it:
> >
> > [   78.804940] BUG: scheduling while atomic: kbd/0x20000000/3444
> > [   78.804944]
> > [   78.804945] Call Trace:
>
> ok, i can think of a simpler scenario:
> add_preempt_count(PREEMPT_ACTIVE) /twice/, nested into each other.

doh - the BKL! That does a down() in a PREEMPT_ACTIVE section, which can
trigger cond_resched(). The fix is to check for PREEMPT_ACTIVE in
cond_resched(). (and only in cond_resched())

Updated fix (against -rc2) attached.

        Ingo

---------------------->
Subject: [patch] sched: fix cond_resched_softirq() offset
From: Ingo Molnar <[email protected]>

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 <[email protected]>
---
 kernel/sched.c |   18 ++++--------------
 1 file changed, 4 insertions(+), 14 deletions(-)

Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -4617,17 +4617,6 @@ asmlinkage long sys_sched_yield(void)
        return 0;
 }

-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;
-}
-
 static void __cond_resched(void)
 {
 #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
@@ -4647,7 +4636,8 @@ 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;
        }
@@ -4673,7 +4663,7 @@ 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();
@@ -4689,7 +4679,7 @@ 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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