From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: suspicious RCU usage warnings in 3.3.0 Date: Wed, 11 Apr 2012 21:54:28 -0700 Message-ID: <20120412045428.GB2497@linux.vnet.ibm.com> References: <20120411171004.016ddd95@nehalam.linuxnetplumber.net> <20120411.201854.1070083308359208025.davem@davemloft.net> <20120412004507.GF2473@linux.vnet.ibm.com> <20120411.210843.716144028821174908.davem@davemloft.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: shemminger@vyatta.com, mroos@linux.ee, linux-kernel@vger.kernel.org, netdev@vger.kernel.org To: David Miller Return-path: Received: from e34.co.us.ibm.com ([32.97.110.152]:47168 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754587Ab2DLEye (ORCPT ); Thu, 12 Apr 2012 00:54:34 -0400 Received: from /spool/local by e34.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Apr 2012 22:54:33 -0600 Content-Disposition: inline In-Reply-To: <20120411.210843.716144028821174908.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, Apr 11, 2012 at 09:08:43PM -0400, David Miller wrote: > From: "Paul E. McKenney" > Date: Wed, 11 Apr 2012 17:45:07 -0700 > > > If I am confused about the simple function call, and if control is really > > passing via an interrupt or exception, then rcu_irq_enter() should be > > called on entry to the interrupt or exception and rcu_irq_exit() should > > be called on exit. > > Hmm, it seems the convention changed such that platforms aren't > supposed to invoke do_softirq() from their trap return trap any more. > It's handled completely by irq_exit(). > > When did that start happening? :-) Heh! It appears that git doesn't go back far enough for me to find the answer to that question. ;-) > Anyways I bet that's the problem, sparc64 invokes do_softirq() in it's > trap return path if softirqs are pending, and that doesn't do any > of the RCU frobbing you mention. The following untested patch that probably does not even build is offered up for your amusement. I don't know enough about SPARC's needs for alignment, handling of branch-delay slots, and so on for this to have any chance of working, but hey! ;-) Thanx, Paul ------------------------------------------------------------------------ sparc64: Eliminate obsolete __handle_softirq() function The invocation of softirq is now handled by irq_exit(), so there is no need for sparc64 to invoke it on the trap-return path. In fact, doing so is a bug because if the trap occurred in the idle loop, this invocation can result in lockdep-RCU failures. The problem is that RCU ignores idle CPUs, and the sparc64 trap-return path to the softirq handlers fails to tell RCU that the CPU must be considered non-idle while those handlers are executing. This means that RCU is ignoring any RCU read-side critical sections in those handlers, which in turn means that RCU-protected data can be yanked out from under those read-side critical sections. The shiny new lockdep-RCU ability to detect RCU read-side critical sections that RCU is ignoring located this problem. The fix is straightforward: Make sparc64 stop manually invoking the softirq handlers. Signed-off-by: Paul E. McKenney --- rtrap_64.S | 7 ------- 1 file changed, 7 deletions(-) diff --git a/arch/sparc/kernel/rtrap_64.S b/arch/sparc/kernel/rtrap_64.S index 77f1b95..9171fc2 100644 --- a/arch/sparc/kernel/rtrap_64.S +++ b/arch/sparc/kernel/rtrap_64.S @@ -20,11 +20,6 @@ .text .align 32 -__handle_softirq: - call do_softirq - nop - ba,a,pt %xcc, __handle_softirq_continue - nop __handle_preemption: call schedule wrpr %g0, RTRAP_PSTATE, %pstate @@ -89,9 +84,7 @@ rtrap: cmp %l1, 0 /* mm/ultra.S:xcall_report_regs KNOWS about this load. */ - bne,pn %icc, __handle_softirq ldx [%sp + PTREGS_OFF + PT_V9_TSTATE], %l1 -__handle_softirq_continue: rtrap_xcall: sethi %hi(0xf << 20), %l4 and %l1, %l4, %l4