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SubjectRe: Realtime Preemption, 2.6.12, Beginners Guide?

* Ingo Molnar <mingo@elte.hu> wrote:

> it worked upon the first try, and indeed my testbox crashed within 10
> seconds:
>
> BUG: Unable to handle kernel NULL pointer dereference
> BUG: Unable to handle kernel NULL pointer dereference at virtual address 00000006

a couple of crashes later i got an important clue:

BUG: bad soft irq-flag value 00000f64, openvpn/3386!
[<c0104052>] dump_stack+0x1f/0x21 (20)
[<c013b883>] check_soft_flags+0x73/0xc9 (24)
[<00000f78>] 0xf78 (1066836133)
it turns out that a small portion of the softirq processing path was
still using the soft IRQ-flag, instead of the raw IRQ-flag! Given enough
irq and softirq workload, we were interrupted in a piece of code where
the data structure was inconsistent. (tinfo.task was already changed,
but %esp not yet) Since interrupts were enabled during the crash
printout, it would crash again and again as it got more interrupts. The
backtrace printout crashed too due to the inconsistency. That's why you
got those repeat ============= lines.
the patch below should fix this bug and i've uploaded the -51-30 patch
with this fix included. Could you check whether 4K stacks are now stable
for you under PREEMPT_RT?

so your intuitition about this being related to 4K stacks was completely
right.

Ingo

Index: linux/arch/i386/kernel/irq.c
===================================================================
--- linux.orig/arch/i386/kernel/irq.c
+++ linux/arch/i386/kernel/irq.c
@@ -169,7 +169,7 @@ asmlinkage void do_softirq(void)
if (in_interrupt())
return;

- local_irq_save(flags);
+ raw_local_irq_save(flags);

if (local_softirq_pending()) {
curctx = current_thread_info();
@@ -190,7 +190,7 @@ asmlinkage void do_softirq(void)
);
}

- local_irq_restore(flags);
+ raw_local_irq_restore(flags);
}

EXPORT_SYMBOL(do_softirq);
-
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