Subject: Re: [v2 PATCH 2/2] booke/kprobe: remove unnecessary preempt_enable_no_resched

From: "tiejun.chen" <[email-masked]>
Date: Mon, 11 Jul 2011 16:34:34 +0800

Ananth N Mavinakayanahalli wrote:
> On Mon, Jul 11, 2011 at 10:39:35AM +0800, Tiejun Chen wrote:
>> When enable CONFIG_PREEMPT we will trigger the following call trace:
>>
>> BUG: scheduling while atomic: swapper/1/0x10000000
>> ...
>>
>> krpobe always goes through the following path:
>>
>> program_check_exception()
>>         |
>>         + notify_die(DIE_BPT, "breakpoint",...)
>>                 |
>>                 + kprobe_handler()
>>                         |
>>                         + preempt_disable();
>>                         + break_handler() <- preempt_enable_no_resched()
>>                         + emulate_step()
>>                         + preempt_enable_no_resched()
>>                         ...
>>         exit
>>
>> We should remove unnecessary preempt_enable_no_resched() inside of break_handler()
>> since looks longjmp_break_handler() always go the above path.
> 
> The current code is correct. Reasoning follows...
> 
> setjmp_pre_handler() and longjmp_break_handler() are used only for
> jprobes. In the case of a jprobe, the code flow would be:
> 
> bp hit -> kprobe_handler() -> preempt_disable() -> setjmp_pre_handler()
> (not that since this routine returns 1, we skip sstep here) -> jp->entry()
> -> jprobe_return() -> bp hit -> kprobe_handler() -> preempt_disable() again
> -> longjmp_break_handler() -> preempt_enable() -> sstep -> preempt_enable()
> (for the second kprobe_handler() entry).
> 
> You could verify this with a preempt_count() printk with a
> CONFIG_PREEMPT=y kernel.
> 
>> Signed-off-by: Tiejun Chen <[email-masked]>
> 
> Nack, sorry :-)

You're right.

When use EXC_LEVEL_EXCEPTION_PROLOG for Critical/Machine check, if the exception
came from kernel mode, we copy thread_info flags, *preempt*, and task pointer
from the process thread_info. So here I steal EXC_LEVEL_EXCEPTION_PROLOG for
Program Exception, preempt count would be corrupted incorrectly.

Thanks
Tiejun

> Ananth