[patch 09/54] Prevent going idle with softirq pending

[Chris Wright]

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

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

 
The NOHZ patch contains a check for softirqs pending when a CPU goes 
idle. The BUG is unrelated to NOHZ, it just was made visible by the NOHZ 
patch. The BUG showed up mainly on P4 / hyperthreading enabled machines 
which lead the investigations into the wrong direction in the first 
place.  The real cause is in cond_resched_softirq():
 
cond_resched_softirq() is enabling softirqs without invoking the softirq 
daemon when softirqs are pending.  This leads to the warning message in 
the NOHZ idle code:
 
t1 runs softirq disabled code on CPU#0
interrupt happens, softirq is raised, but deferred (softirqs disabled)
t1 calls cond_resched_softirq()
	enables softirqs via _local_bh_enable()
	calls schedule()
t2 runs
t1 is migrated to CPU#1
t2 is done and invokes idle()
NOHZ detects the pending softirq
 
Fix: change _local_bh_enable() to local_bh_enable() so the softirq
daemon is invoked.
 
Thanks to Anant Nitya for debugging this with great patience !
 
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Anant Nitya <kernel@xxxxxxxxxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
 kernel/sched.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

--- linux-2.6.21.4.orig/kernel/sched.c
+++ linux-2.6.21.4/kernel/sched.c
@@ -4545,9 +4545,7 @@ int __sched cond_resched_softirq(void)
 	BUG_ON(!in_softirq());
 
 	if (need_resched() && system_state == SYSTEM_RUNNING) {
-		raw_local_irq_disable();
-		_local_bh_enable();
-		raw_local_irq_enable();
+		local_bh_enable();
 		__cond_resched();
 		local_bh_disable();
 		return 1;

-- 
-
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