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

From: Chris Wright
Date: Fri Jun 08 2007 - 03:43:37 EST

-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-
+++ linux-
@@ -4545,9 +4545,7 @@ int __sched cond_resched_softirq(void)

if (need_resched() && system_state == SYSTEM_RUNNING) {
- raw_local_irq_disable();
- _local_bh_enable();
- raw_local_irq_enable();
+ local_bh_enable();
return 1;

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