[PATCH] Prevent going idle with softirq pending

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@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Anant Nitya <kernel@prachanda.info>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/sched.c b/kernel/sched.c
index b9a6837301482270dc7438a3df3539b9ac835f94..a3993b9e7503a9aa033dc398989845388fa3e3de 100644 (file)
--- a/kernel/sched.c
+++ b/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;