Re: [PATCH -tip 4/4] Atomic text_poke() with fixmap take2

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

* 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.

Mathieu


-- 
Mathieu Desnoyers
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