Re: [RFC][PATCH] x86: make text_poke() atomic

[Masami Hiramatsu <mhiramat@redhat.com>]

Ingo Molnar wrote:
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
> 
>> Mathieu Desnoyers wrote:
>>> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
>>>> Index: linux-2.6/init/main.c
>>>> ===================================================================
>>>> --- linux-2.6.orig/init/main.c
>>>> +++ linux-2.6/init/main.c
>>>> @@ -676,6 +676,9 @@ asmlinkage void __init start_kernel(void
>>>>  	taskstats_init_early();
>>>>  	delayacct_init();
>>>>
>>>> +#ifdef CONFIG_X86
>>>> +	text_poke_init();
>>>> +#endif
>>> All good, except this above. There should be an empty text_poke_init()
>>> in some header file, and an implementation for the X86 arch rather than
>>> a ifdef in init/main.c.
>> Hmm, I'd rather use __weak function instead of defining it in some header
>> files, because text_poke() and alternatives exist only on x86.
>>
>> I know that we need to discuss cross modifying code on x86 with
>> Arjan or other Intel engineers. This patch may still be useful
>> for removing unnecessary vm_area allocation in text_poke().
>>
>> Thank you,
>>
>> ---
>>
>> Use map_vm_area() instead of vmap() in text_poke() for 
>> avoiding page allocation and delayed unmapping, and call 
>> vunmap_page_range() and local_flush_tlb() directly because 
>> this mapping is temporary and local.
>>
>> At the result of above change, text_poke() becomes atomic and 
>> can be called from stop_machine() etc.
> 
> That looks like a good fix in itself - see a few minor details 
> below.

Thank you for review,

> 
> (Note, i could not try your patch because it has widespread 
> whitespace damage - please watch out for this for future 
> patches.)

Oops, it was my mis-setting...

> 
>> +static struct vm_struct *text_poke_area[2];
>> +static DEFINE_SPINLOCK(text_poke_lock);
>> +
>> +void __init text_poke_init(void)
>> +{
>> +	text_poke_area[0] = get_vm_area(PAGE_SIZE, VM_ALLOC);
>> +	text_poke_area[1] = get_vm_area(2 * PAGE_SIZE, VM_ALLOC);
>> +	BUG_ON(!text_poke_area[0] || !text_poke_area[1]);
> 
> BUG_ON() for non-100%-essential init code is a no-no. Please 
> change it to WARN_ON() so that people have a chance to report i.

Sure.

> 
> Also, i think all these vma complications came from the decision 
> to use vmap - and vmap enhancements in .29 complicated this 
> supposedly-simple interface.
> 
> So perhaps another approach to (re-)consider would be to go back 
> to atomic fixmaps here. It spends 3 slots but that's no big 
> deal.

Oh, it's a good idea! fixmaps must make it simpler.

> 
> In exchange it will be conceptually simpler, and will also scale 
> much better than a global spinlock. What do you think?

I think even if I use fixmaps, we have to use a spinlock to protect
the fixmap area from other threads...

> 
> 	Ingo
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-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com