From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756899Ab2GEQSy (ORCPT ); Thu, 5 Jul 2012 12:18:54 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:44989 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751812Ab2GEQSw (ORCPT ); Thu, 5 Jul 2012 12:18:52 -0400 Date: Thu, 5 Jul 2012 09:16:44 -0700 From: "Paul E. McKenney" To: mingo@elte.hu Cc: linux-kernel@vger.kernel.org, levinsasha928@gmail.com Subject: [GIT RFC PULL rcu/urgent] Revert to fix RCU-related deadlock/softlockup Message-ID: <20120705161644.GA10670@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12070516-5518-0000-0000-000005CB66BA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Ingo, This series has a single revert from the ill-starred attempt to inline __rcu_read_lock() for preemptible RCU. Without this revert, on mainline kernels using CONFIG_RCU_BOOST there is a low-probability deadlock on the runqueue locks, but one that actually appeared in Sasha Levin's testing. With the revert, and with an diagnostic patch that increased probability of the deadlock to a MTBF of roughly 10 seconds, Sasha's tests ran for two days with no failure. The sequence of events leading to the deadlock is as follows: 1. A task enters an RCU read-side critical section, and is both preempted and subjected to RCU priority boosting. 2. The task starts to exit its RCU read-side critical section, but is preempted in __rcu_read_unlock() just after the assignment setting t->rcu_read_lock_nesting to INT_MIN. (The diagnostic patch mentioned above expands this window by ten microseconds, and is available in -rcu as a debug option queued for 3.7.) 3. The task enters the scheduler, where it acquires the corresponding runqueue lock, then invokes rcu_switch_from() which in turn invokes rcu_preempt_note_context_switch(), which in turn invokes rcu_read_unlock_special(), which attempts to deboost the task. 4. The attempt to deboost the task recursively enters the scheduler with a runqueue lock held, which can result in deadlock. The revert moves the point at which rcu_preempt_note_context_switch() is called to a point in the scheduler code before the runqueue lock is acquired, avoiding the deadlock. This pull is marked "RFC" because CONFIG_RCU_BOOST=y is not used much outside of the real-time community. I will be sending another pull request later today (Pacific Time) for 3.6 RCU commits, which will include this commit as well. Your choice. ;-) This change is available in the git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/urgent Thanx, Paul ------------------> Paul E. McKenney (1): Revert "rcu: Move PREEMPT_RCU preemption to switch_to() invocation" arch/um/drivers/mconsole_kern.c | 1 - include/linux/rcupdate.h | 1 - include/linux/rcutiny.h | 6 ++++++ include/linux/sched.h | 10 ---------- kernel/rcutree.c | 1 + kernel/rcutree.h | 1 + kernel/rcutree_plugin.h | 14 +++++++++++--- kernel/sched/core.c | 1 - 8 files changed, 19 insertions(+), 16 deletions(-)