From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752641Ab3BQTlY (ORCPT ); Sun, 17 Feb 2013 14:41:24 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:38437 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752494Ab3BQTlW (ORCPT ); Sun, 17 Feb 2013 14:41:22 -0500 Date: Sat, 16 Feb 2013 12:59:09 -0800 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: Steven Rostedt , linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, Carsten Emde , Thomas Gleixner Subject: Re: [PATCH 04/16] rcu: rcutiny: Prevent RCU stall Message-ID: <20130216205909.GB3094@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1360771932-27150-1-git-send-email-bigeasy@linutronix.de> <1360771932-27150-5-git-send-email-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1360771932-27150-5-git-send-email-bigeasy@linutronix.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13021719-9360-0000-0000-000010B44BEA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 13, 2013 at 05:11:59PM +0100, Sebastian Andrzej Siewior wrote: > From: Thomas Gleixner > > rcu_read_unlock_special() checks in_serving_softirq() and leaves early > when true. On RT this is obviously wrong as softirq processing context > can be preempted and therefor such a task can be on the gp_tasks > list. Leaving early here will leave the task on the list and therefor > block RCU processing forever. > > This cannot happen on mainline because softirq processing context > cannot be preempted and therefor this can never happen at all. > > In fact this check looks quite questionable in general. Neither irq > context nor softirq processing context in mainline can ever be > preempted in mainline so the special unlock case should not ever be > invoked in such context. Now the only explanation might be a > rcu_read_unlock() being interrupted and therefor leave the rcu nest > count at 0 before the special unlock bit has been cleared. That looks > fragile. At least it's missing a big fat comment. Paul ???? > > See mainline commits: ec433f0c5 and 8762705a for further enlightment. > > Reported-by: Kristian Lehmann > Signed-off-by: Thomas Gleixner > [bigeasy@linutronix: different in-irq check] > Signed-off-by: Sebastian Andrzej Siewior > --- > kernel/rcutiny_plugin.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h > index 2b0484a..bac1906 100644 > --- a/kernel/rcutiny_plugin.h > +++ b/kernel/rcutiny_plugin.h > @@ -552,7 +552,7 @@ static void rcu_read_unlock_special(struct task_struct *t) > rcu_preempt_cpu_qs(); > > /* Hardware IRQ handlers cannot block. */ > - if (in_irq()) { > + if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) { For whatever it is worth, in mainline this is: if (in_irq() || in_serving_softirq()) { The definition of in_serving_softirq() is a bit different: #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET) This might be due to differences between mainline and -rt, but thought it worth calling attention to. Thanx, Paul > local_irq_restore(flags); > return; > } > -- > 1.7.10.4 > > -- > 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/ >