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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Ingo Molnar wrote:
> > * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> > 
> >> * Ingo Molnar (mingo@elte.hu) wrote:
> >>> * Masami Hiramatsu <mhiramat@redhat.com> 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@redhat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> Cc: Ingo Molnar <mingo@elte.hu>
> ---
>  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]);

I think irq off should cover the BUG_ON too. This safety check assumes
we are the only ones modifying "addr".

Mathieu

>  	return addr;
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@redhat.com
> 

-- 
Mathieu Desnoyers
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