Subject: softirq: Init softirq local lock after per cpu section is set up
From: Steven Rostedt <rostedt@goodmis.org>
Date: Thu, 04 Oct 2012 11:02:04 -0400

I discovered this bug when booting 3.4-rt on my powerpc box. It crashed
with the following report:

------------[ cut here ]------------
kernel BUG at /work/rt/stable-rt.git/kernel/rtmutex_common.h:75!
Oops: Exception in kernel mode, sig: 5 [#1]
PREEMPT SMP NR_CPUS=64 NUMA PA Semi PWRficient
Modules linked in:
NIP: c0000000004aa03c LR: c0000000004aa01c CTR: c00000000009b2ac
REGS: c00000003e8d7950 TRAP: 0700   Not tainted  (3.4.11-test-rt19)
MSR: 9000000000029032 <SF,HV,EE,ME,IR,DR,RI>  CR: 24000082  XER: 20000000
SOFTE: 0
TASK = c00000003e8fdcd0[11] 'ksoftirqd/1' THREAD: c00000003e8d4000 CPU: 1
GPR00: 0000000000000001 c00000003e8d7bd0 c000000000d6cbb0 0000000000000000 
GPR04: c00000003e8fdcd0 0000000000000000 0000000024004082 c000000000011454 
GPR08: 0000000000000000 0000000080000001 c00000003e8fdcd1 0000000000000000 
GPR12: 0000000024000084 c00000000fff0280 ffffffffffffffff 000000003ffffad8 
GPR16: ffffffffffffffff 000000000072c798 0000000000000060 0000000000000000 
GPR20: 0000000000642741 000000000072c858 000000003ffffaf0 0000000000000417 
GPR24: 000000000072dcd0 c00000003e7ff990 0000000000000000 0000000000000001 
GPR28: 0000000000000000 c000000000792340 c000000000ccec78 c000000001182338 
NIP [c0000000004aa03c] .wakeup_next_waiter+0x44/0xb8
LR [c0000000004aa01c] .wakeup_next_waiter+0x24/0xb8
Call Trace:
[c00000003e8d7bd0] [c0000000004aa01c] .wakeup_next_waiter+0x24/0xb8 (unreliable)
[c00000003e8d7c60] [c0000000004a0320] .rt_spin_lock_slowunlock+0x8c/0xe4
[c00000003e8d7ce0] [c0000000004a07cc] .rt_spin_unlock+0x54/0x64
[c00000003e8d7d60] [c0000000000636bc] .__thread_do_softirq+0x130/0x174
[c00000003e8d7df0] [c00000000006379c] .run_ksoftirqd+0x9c/0x1a4
[c00000003e8d7ea0] [c000000000080b68] .kthread+0xa8/0xb4
[c00000003e8d7f90] [c00000000001c2f8] .kernel_thread+0x54/0x70
Instruction dump:
60000000 e86d01c8 38630730 4bff7061 60000000 ebbf0008 7c7c1b78 e81d0040 
7fe00278 7c000074 7800d182 68000001 <0b000000> e88d01c8 387d0010 38840738 

The rtmutex_common.h:75 is:

rt_mutex_top_waiter(struct rt_mutex *lock)
{
	struct rt_mutex_waiter *w;

	w = plist_first_entry(&lock->wait_list, struct rt_mutex_waiter,
			       list_entry);
	BUG_ON(w->lock != lock);

	return w;
}

Where the waiter->lock is corrupted. I saw various other random bugs
that all had to with the softirq lock and plist. As plist needs to be
initialized before it is used I investigated how this lock is
initialized. It's initialized with:

void __init softirq_early_init(void)
{
	local_irq_lock_init(local_softirq_lock);
}

Where:

#define local_irq_lock_init(lvar)					\
	do {								\
		int __cpu;						\
		for_each_possible_cpu(__cpu)				\
			spin_lock_init(&per_cpu(lvar, __cpu).lock);	\
	} while (0)

As the softirq lock is a local_irq_lock, which is a per_cpu lock, the
initialization is done to all per_cpu versions of the lock. But lets
look at where the softirq_early_init() is called from.

In init/main.c: start_kernel()

/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
	softirq_early_init();
	tick_init();
	boot_cpu_init();
	page_address_init();
	printk(KERN_NOTICE "%s", linux_banner);
	setup_arch(&command_line);
	mm_init_owner(&init_mm, &init_task);
	mm_init_cpumask(&init_mm);
	setup_command_line(command_line);
	setup_nr_cpu_ids();
	setup_per_cpu_areas();
	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */

One of the first things that is called is the initialization of the
softirq lock. But if you look further down, we see the per_cpu areas
have not been set up yet. Thus initializing a local_irq_lock() before
the per_cpu section is set up, may not work as it is initializing the
per cpu locks before the per cpu exists.

By moving the softirq_early_init() right after setup_per_cpu_areas(),
the kernel boots fine.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Clark Williams <clark@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Carsten Emde <cbe@osadl.org>
Cc: vomlehn@texas.net
Link: http://lkml.kernel.org/r/1349362924.6755.18.camel@gandalf.local.home
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 init/main.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-stable/init/main.c
===================================================================
--- linux-stable.orig/init/main.c
+++ linux-stable/init/main.c
@@ -491,7 +491,6 @@ asmlinkage void __init start_kernel(void
  * Interrupts are still disabled. Do necessary setups, then
  * enable them
  */
-	softirq_early_init();
 	tick_init();
 	boot_cpu_init();
 	page_address_init();
@@ -502,6 +501,7 @@ asmlinkage void __init start_kernel(void
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
 	setup_per_cpu_areas();
+	softirq_early_init();
 	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
 
 	build_all_zonelists(NULL, NULL);