From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262496AbVCLX3v (ORCPT ); Sat, 12 Mar 2005 18:29:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262519AbVCLXZd (ORCPT ); Sat, 12 Mar 2005 18:25:33 -0500 Received: from bay-bridge.veritas.com ([143.127.3.10]:14273 "EHLO MTVMIME03.enterprise.veritas.com") by vger.kernel.org with ESMTP id S262393AbVCLXWX (ORCPT ); Sat, 12 Mar 2005 18:22:23 -0500 Date: Sat, 12 Mar 2005 23:20:53 +0000 (GMT) From: Hugh Dickins X-X-Sender: hugh@goblin.wat.veritas.com To: Andrew Morton cc: mingo@elte.hu, nickpiggin@yahoo.com.au, linux-kernel@vger.kernel.org Subject: Re: [PATCH] break_lock forever broken In-Reply-To: <20050311203427.052f2b1b.akpm@osdl.org> Message-ID: References: <20050311203427.052f2b1b.akpm@osdl.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 11 Mar 2005, Andrew Morton wrote: > > This patch causes a CONFIG_PREEMPT=y, CONFIG_PREEMPT_BKL=y, > CONFIG_DEBUG_PREEMPT=y kernel on a ppc64 G5 to hang immediately after > displaying the penguins, but apparently not before having set the hardware > clock backwards 101 years. > > After having carefully reviewed the above description and having decided > that these effects were not a part of the patch's design intent I have > temporarily set it aside, thanks. That was a Klingon patch, sorry, it escaped. Hence the time warp. Did terrible things on i386 too once I tested it differently. When dealing with something like a completion, disaster to touch a field of the lock once it's been unlocked. Here's a replacement... lock->break_lock is set when a lock is contended, but cleared only in cond_resched_lock. Users of need_lockbreak (journal_commit_transaction, copy_pte_range, unmap_vmas) don't necessarily use cond_resched_lock on it. So, if the lock has been contended at some time in the past, break_lock remains set thereafter, and the fastpath keeps dropping lock unnecessarily. Hanging the system if you make a change like I did, forever restarting a loop before making any progress. And even users of cond_resched_lock may well suffer an initial unnecessary lockbreak. There seems to be no point at which break_lock can be cleared when unlocking, any point being either too early or too late; but that's okay, it's only of interest while the lock is held. So clear it whenever the lock is acquired - and any waiting contenders will quickly set it again. Additional locking overhead? well, this is only when CONFIG_PREEMPT is on. Since cond_resched_lock's spin_lock clears break_lock, no need to clear it itself; and use need_lockbreak there too, preferring optimizer to #ifdefs. Signed-off-by: Hugh Dickins --- 2.6.11-bk8/kernel/sched.c 2005-03-11 13:33:09.000000000 +0000 +++ linux/kernel/sched.c 2005-03-11 17:46:50.000000000 +0000 @@ -3753,14 +3753,11 @@ EXPORT_SYMBOL(cond_resched); */ int cond_resched_lock(spinlock_t * lock) { -#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT) - if (lock->break_lock) { - lock->break_lock = 0; + if (need_lockbreak(lock)) { spin_unlock(lock); cpu_relax(); spin_lock(lock); } -#endif if (need_resched()) { _raw_spin_unlock(lock); preempt_enable_no_resched(); --- 2.6.11-bk8/kernel/spinlock.c 2005-03-02 07:38:52.000000000 +0000 +++ linux/kernel/spinlock.c 2005-03-12 22:52:41.000000000 +0000 @@ -187,6 +187,8 @@ void __lockfunc _##op##_lock(locktype##_ cpu_relax(); \ preempt_disable(); \ } \ + if ((lock)->break_lock) \ + (lock)->break_lock = 0; \ } \ \ EXPORT_SYMBOL(_##op##_lock); \ @@ -209,6 +211,8 @@ unsigned long __lockfunc _##op##_lock_ir cpu_relax(); \ preempt_disable(); \ } \ + if ((lock)->break_lock) \ + (lock)->break_lock = 0; \ return flags; \ } \ \