Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933395AbWKSVvE (ORCPT ); Sun, 19 Nov 2006 16:51:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933400AbWKSVvD (ORCPT ); Sun, 19 Nov 2006 16:51:03 -0500 Received: from host-233-54.several.ru ([213.234.233.54]:24811 "EHLO mail.screens.ru") by vger.kernel.org with ESMTP id S933395AbWKSVvB (ORCPT ); Sun, 19 Nov 2006 16:51:01 -0500 Date: Mon, 20 Nov 2006 00:50:53 +0300 From: Oleg Nesterov To: "Paul E. McKenney" Cc: Alan Stern , Jens Axboe , Linus Torvalds , Thomas Gleixner , Ingo Molnar , LKML , john stultz , David Miller , Arjan van de Ven , Andrew Morton , Andi Kleen , manfred@colorfullife.com Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync Message-ID: <20061119215053.GA176@oleg> References: <20061119190027.GA3676@oleg> <20061119205516.GA117@oleg> <20061119212057.GE4427@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20061119212057.GE4427@us.ibm.com> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2168 Lines: 68 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/