[PATCH -tip 5/4] Expands irq-off region in text_poke()

[Masami Hiramatsu]

Ingo Molnar wrote:
> * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
> 
>> * Ingo Molnar (mingo@xxxxxxx) wrote:
>>> * Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>>>
>>>> @@ -523,14 +526,17 @@ void *__kprobes text_poke(void *addr, co
>>>>  		pages[1] = virt_to_page(addr + PAGE_SIZE);
>>>>  	}
>>>>  	BUG_ON(!pages[0]);
>>>> -	if (!pages[1])
>>>> -		nr_pages = 1;
>>>> -	vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
>>>> -	BUG_ON(!vaddr);
>>>> -	local_irq_disable();
>>>> +	local_irq_save(flags);
>>>> +	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
>>>> +	if (pages[1])
>>>> +		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
>>>> +	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
>>>>  	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
>>>> -	local_irq_enable();
>>>> -	vunmap(vaddr);
>>>> +	clear_fixmap(FIX_TEXT_POKE0);
>>>> +	if (pages[1])
>>>> +		clear_fixmap(FIX_TEXT_POKE1);
>>>> +	local_flush_tlb();
>>>> +	local_irq_restore(flags);
>>>>  	sync_core();
>>> I'm not sure at all about this widening of the irq-atomic 
>>> section and the idea of allowing non-locked access on single-CPU 
>>> situations - we dont really want to micro-optimize any of this 
>>> on such a level, holding the text lock is a robust rule all code 
>>> should be listening to. (Creating locking assymetry always 
>>> inserts a certain amount of fragility - adding to an already 
>>> fragile concept here.)
>>>
>>> And note that there's no reason why text_poke could not be used 
>>> in stop_machine_run() - the stop_machine_run() handler must not 
>>> take the text_lock of course - but outside code calling 
>>> stop_machine_run() can do it and can hence serialize properly.
>>>
>>> Note that even if we did this then your v2 patch is not fully 
>>> correct: you need to move the sync_core() at the end of the 
>>> sequence inside the critical section too. (right now this is 
>>> mostly harmless because the INVLPG inside the clear_fixmap() 
>>> happens to be serializing so it has an implicit sync_core() 
>>> property - but nevertheless we better do this straight away to 
>>> not cause problems later down the line.)
>>>
>>> 	Ingo
>> Agreed. The alternatives_smp_lock/alternatives_smp_unlock 
>> specific case does not bring us much if it has no perceivable 
>> performance impact. It's better to keep a standard interface 
>> and clear requirements.
> 
> Note that i dont object to another aspect of this same change: 
> the fact that it makes the whole sequence more atomic and more 
> defensive [which is never bad of fragile interfaces].
> 
> I only got worried about the "lets use this without the text 
> lock" ideas.
> 
> So if Masami-san sends a delta patch with a different changelog 
> and with the sync_core() bit moved inside the critical section, 
> i'll apply that too.

OK, here is the delta patch.

Expand irq-atomic region to cover fixmap using code and sync_core.

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
---
 arch/x86/kernel/alternative.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-2.6-tip/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6-tip.orig/arch/x86/kernel/alternative.c
+++ linux-2.6-tip/arch/x86/kernel/alternative.c
@@ -526,13 +526,12 @@ void *__kprobes text_poke(void *addr, co
 		pages[1] = virt_to_page(addr + PAGE_SIZE);
 	}
 	BUG_ON(!pages[0]);
+	local_irq_save(flags);
 	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
 	if (pages[1])
 		set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
 	vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
-	local_irq_save(flags);
 	memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
-	local_irq_restore(flags);
 	clear_fixmap(FIX_TEXT_POKE0);
 	if (pages[1])
 		clear_fixmap(FIX_TEXT_POKE1);
@@ -540,6 +539,7 @@ void *__kprobes text_poke(void *addr, co
 	sync_core();
 	/* Could also do a CLFLUSH here to speed up CPU recovery; but
 	   that causes hangs on some VIA CPUs. */
+	local_irq_restore(flags);
 	for (i = 0; i < len; i++)
 		BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
 	return addr;
-- 
Masami Hiramatsu

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

e-mail: mhiramat@xxxxxxxxxx

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