linux-kernel - Re: [Kgdb-bugreport] [GIT PULL] kgdb regression fixes for 2.6.35-rc5

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Date:	Thu, 29 Jul 2010 10:49:50 +0800
From:	DDD <dongdong.deng@...driver.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Jason Wessel <jason.wessel@...driver.com>,
	kgdb-bugreport@...ts.sourceforge.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [Kgdb-bugreport] [GIT PULL] kgdb regression fixes for 2.6.35-rc5

Frederic Weisbecker wrote:
> On Wed, Jul 28, 2010 at 07:14:42PM -0500, Jason Wessel wrote:
>> On 07/28/2010 04:26 PM, Linus Torvalds wrote:
>>> On Wed, Jul 28, 2010 at 2:17 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
>>>> I'm sorry I spot it a bit late.
>>>> There is a little issue in this patch, irqs won't
>>>> be restored:
>>>>
>>>> @@ -588,9 +588,10 @@ int kgdb_ll_trap(int cmd, const char *str,
>>>>        if (!kgdb_io_module_registered)
>>>>                return NOTIFY_DONE;
>>>>
>>>> +       local_irq_save(flags);
>>>>        return __kgdb_notify(&args, cmd);
>>>> +       local_irq_restore(flags);
>>>>  }
>>> Yeah, that's obviously crap. I also wonder why __kgdb_notify messes up
>>> the flags to begin with.
>>>
>> This should not be needed as the irqs should already be off while in the overflow handler.
> 
> 
> 
> Interrupts are not disabled in the debug exception handler.

Hi Frederic,

The debug was initialized as an intr gate, thus the interrupts have been 
disabled in the debug exception handler. :-)

arch/x86/kernel/traps.c:
820 /* Set of traps needed for early debugging. */
821 void __init early_trap_init(void)
822 {
823     set_intr_gate_ist(1, &debug, DEBUG_STACK);
824     /* int3 can be called from all */
825     set_system_intr_gate_ist(3, &int3, DEBUG_STACK);
826     set_intr_gate(14, &page_fault);
827     load_idt(&idt_descr);
828 }

Thanks,
Dongdong

> 
> I'm not sure what would be the state of dr6 if a nested debug exception
> would happen while do_debug() is interrupted.
> 
> Perhaps the previous dr6 value is dropped. Intel manuals don't seem to say anything
> about that.
> 
> Anyway that's a separate issue. The current patch fixes a regression and
> it looks good.
> 
> Thanks.
> 
> 
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