linux-kernel - Re: [PATCH -tip 4/4] Atomic text

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Date:	Fri, 6 Mar 2009 20:22:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip 4/4] Atomic text_poke() with fixmap take2


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> * Ingo Molnar (mingo@...e.hu) wrote:
> > 
> > * Masami Hiramatsu <mhiramat@...hat.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.

	Ingo
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