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

From: "tiejun.chen" <[email-masked]>
Date: Mon, 11 Jul 2011 19:28:31 +0800

tiejun.chen wrote:
> 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.

Looks I miss the specific return-from-program-exc to restore those necessary
thread information like we did for debug exception with ret_from_debug_exc when
use EXC_LEVEL_EXCEPTION_PROLOG for debug exception.

Will update this on v3.

Tiejun

> 
> Thanks
> Tiejun
> 
>> Ananth
>>
> 
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