lockdep: results on ARM

From: Russell King
Date: Thu Jul 27 2006 - 10:22:29 EST


I've been working on getting lockdep up and running on ARM, and have hit
a problem wrt our context switching with IRQs enabled.

I'm seeing:

Calibrating local timer... 104.40MHz.
BUG: warning at /home/rmk/git/linux-2.6-rmk/kernel/lockdep.c:2343/check_flags()
[<c0024ed0>] (dump_stack+0x0/0x14) from [<c005cf20>] (check_flags+0x88/0x1e4)
[<c005ce98>] (check_flags+0x0/0x1e4) from [<c005d0c4>] (lock_acquire+0x48/0x94)
r5 = C05D4000 r4 = 60000013
[<c005d07c>] (lock_acquire+0x0/0x94) from [<c01e0f70>] (_spin_lock+0x30/0x40)
[<c01e0f40>] (_spin_lock+0x0/0x40) from [<c0041e5c>] (exit_fs+0x28/0xbc)
r4 = C05CD26C
[<c0041e34>] (exit_fs+0x0/0xbc) from [<c0055900>] (kthread+0x2c/0x110)
r6 = C021F520 r5 = C05CD040 r4 = C05D4000
[<c00558d4>] (kthread+0x0/0x110) from [<c00423b8>] (do_exit+0x0/0x83c)
r8 = 00000000 r7 = 00000000 r6 = 00000000 r5 = 00000000
r4 = 00000000
irq event stamp: 0
hardirqs last enabled at (0): [<00000000>] 0x0
hardirqs last disabled at (0): [<c003d918>] copy_process+0x31c/0x1274
softirqs last enabled at (0): [<c003d918>] copy_process+0x31c/0x1274
softirqs last disabled at (0): [<00000000>] 0x0
CPU1: Booted secondary processor

which appears to be the lockdep code complaining that IRQs are on when
it thinks they shouldn't be.

Since new processes are created with the lockdep irq state marked as
"disabled", and the scheduler does not change IRQ state if an architecture
sets __ARCH_WANT_INTERRUPTS_ON_CTXSW, there is nothing which causes
lockdep to think IRQs should be on. Moreover, trying to call the
trace_hardirqs_on() function doesn't work because it then bitches that
IRQs are already on.

The best fix I've come up with which seems to work is:

diff --git a/kernel/fork.c b/kernel/fork.c
index 1b0f7b1..bc8cda2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1055,7 +1055,11 @@ #ifdef CONFIG_NUMA
p->irq_events = 0;
+ p->hardirqs_enabled = 1;
p->hardirqs_enabled = 0;
p->hardirq_enable_ip = 0;
p->hardirq_enable_event = 0;
p->hardirq_disable_ip = _THIS_IP_;

Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core
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