linux-kernel - Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall

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Date:	Mon, 20 Nov 2006 00:50:53 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ibm.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...l.org>,
	Thomas Gleixner <tglx@...esys.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	David Miller <davem@...emloft.net>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>,
	manfred@...orfullife.com
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On 11/19, Paul E. McKenney wrote:
>
> On Sun, Nov 19, 2006 at 11:55:16PM +0300, Oleg Nesterov wrote:
> > So synchronize_xxx() should do
> >
> > 	smp_mb();
> > 	idx = sp->completed++ & 0x1;
> >
> > 	for (;;) { ... }
> >
> > >               You see, there's no way around using synchronize_sched().
> >
> > With this change I think we are safe.
> >
> > If synchronize_xxx() increments ->completed in between, the caller of
> > xxx_read_lock() will see all memory ops (started before synchronize_xxx())
> > completed. It is ok that synchronize_xxx() returns immediately.
>
> Let me take Alan's example one step further:
>
> o	CPU 0 starts executing xxx_read_lock(), but is interrupted
> 	(or whatever) just before the atomic_inc().
>
> o	CPU 1 executes synchronize_xxx() to completion, which it
> 	can because CPU 0 has not yet incremented the counter.

Let's suppose for simplicity that CPU 1 does "classical"

	old = global_ptr;
	global_ptr = new_value();

before synchronize_xxx(), and ->completed == 0.

Now, synchronize_xxx() sets ->completed == 1. Because of mb()
'global_ptr = new_value()' is completed.

> o	CPU 0 returns from interrupt and completes xxx_read_lock(),
> 	but has incremented the wrong counter.

->completed == 1, it is not so wrong, see below

> o	CPU 0 continues into its critical section, picking up a
> 	pointer to an xxx-protected data structure (or, in Jens's
> 	case starting an xxx-protected I/O).

it sees the new value in global_ptr, we are safe.

> o	CPU 1 executes another synchronize_xxx().  This completes
> 	immediately because CPU 1 has the wrong counter incremented.

No, it will notice .ctr[1] != 1 and wait.

> o	CPU 1 continues, either freeing a data structure while
> 	CPU 0 is still referencing it, or, in Jens's case, completing
> 	an I/O barrier while there is still outstanding I/O.

CPU 1 continues only when CPU 0 does read_unlock(/*completed*/ 1),
we are safe.

Safe?

Oleg.

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