linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Date:	Thu, 13 Nov 2008 13:35:20 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function
 return tracer lockless


On Thu, 13 Nov 2008, Andi Kleen wrote:

> On Thu, Nov 13, 2008 at 12:32:23PM -0500, Steven Rostedt wrote:
> > 
> > On Thu, 13 Nov 2008, Andi Kleen wrote:
> > 
> > > > So the answer to this is:
> > > > 
> > > >   i = index++;
> > > >   barrier();
> > > >   write to index i (not index);
> > > 
> > > That was my first thought when I wrote the original email,
> > > but the disadvantage is that barrier() is a big hammer
> > > that flushes everything and can make the code much worse.
> > > That is why I suggested local_add_return() instead.
> > 
> > barrier() is a compiler barrier, does nothing with the caches, and is 
> > quite cheap. We only need a compiler barrier because we are only 
> 
> I did not refer to CPU caches, but the compiler's register allocation
> [ok if you want the registers are the "level 0 cache"]. A memory barrier
> all messes it up. That is why it is better to only clobber specific
> memory regions, which is what local_* does.
> 


#define barrier() __asm__ __volatile__("": : :"memory")

static inline long local_add_return(long i, local_t *l)
{
	long __i;
#ifdef CONFIG_M386
	unsigned long flags;
	if (unlikely(boot_cpu_data.x86 <= 3))
		goto no_xadd;
#endif
	/* Modern 486+ processor */
	__i = i;
	asm volatile(_ASM_XADD "%0, %1;"
		     : "+r" (i), "+m" (l->a.counter)
		     : : "memory");
	return i + __i;

#ifdef CONFIG_M386
no_xadd: /* Legacy 386 processor */
	local_irq_save(flags);
	__i = local_read(l);
	local_set(l, i + __i);
	local_irq_restore(flags);
	return i + __i;
#endif
}



Now tell me again how local_* is more efficient than barrier?

Not to mention, if this is ever used on other archs with load-linked and 
store-conditional, it gets even worse.

-- Steve

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