From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757736Ab2AKQu7 (ORCPT ); Wed, 11 Jan 2012 11:50:59 -0500 Received: from e7.ny.us.ibm.com ([32.97.182.137]:60388 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757146Ab2AKQuz (ORCPT ); Wed, 11 Jan 2012 11:50:55 -0500 Date: Wed, 11 Jan 2012 08:11:53 -0800 From: "Paul E. McKenney" To: "Srivatsa S. Bhat" Cc: bp@amd64.org, tony.luck@intel.com, tglx@linutronix.de, mingo@elte.hu, hpa@zytor.com, x86@kernel.org, linux-edac@vger.kernel.org, linux-kernel@vger.kernel.org, marcos.mage@gmail.com, prasad@linux.vnet.ibm.com Subject: Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer() Message-ID: <20120111161153.GA2925@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120111142213.16610.85866.stgit@srivatsabhat.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120111142213.16610.85866.stgit@srivatsabhat.in.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) x-cbid: 12011116-5806-0000-0000-00001149E041 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 11, 2012 at 07:54:48PM +0530, Srivatsa S. Bhat wrote: > While booting, the following message is seen: Hmmm... This code is interesting. It looks to me that access to each entry is gated by the ->finished field. If so, there should be no need for any kind of rcu_dereference() on mcelog.next -- ACCESS_ONCE() should work just fine. > [ 21.665087] =============================== > [ 21.669439] [ INFO: suspicious RCU usage. ] > [ 21.673798] 3.2.0-0.0.0.28.36b5ec9-default #2 Not tainted > [ 21.681353] ------------------------------- > [ 21.685864] arch/x86/kernel/cpu/mcheck/mce.c:194 suspicious rcu_dereference_index_check() usage! > [ 21.695013] > [ 21.695014] other info that might help us debug this: > [ 21.695016] > [ 21.703488] > [ 21.703489] rcu_scheduler_active = 1, debug_locks = 1 > [ 21.710426] 3 locks held by modprobe/2139: > [ 21.714754] #0: (&__lockdep_no_validate__){......}, at: [] __driver_attach+0x53/0xa0 > [ 21.725020] #1: > [ 21.725323] ioatdma: Intel(R) QuickData Technology Driver 4.00 > [ 21.733206] (&__lockdep_no_validate__){......}, at: [] __driver_attach+0x61/0xa0 > [ 21.743015] #2: (i7core_edac_lock){+.+.+.}, at: [] i7core_probe+0x1f/0x5c0 [i7core_edac] > [ 21.753708] > [ 21.753709] stack backtrace: > [ 21.758429] Pid: 2139, comm: modprobe Not tainted 3.2.0-0.0.0.28.36b5ec9-default #2 > [ 21.768253] Call Trace: > [ 21.770838] [] lockdep_rcu_suspicious+0xcd/0x100 > [ 21.777366] [] drain_mcelog_buffer+0x191/0x1b0 > [ 21.783715] [] mce_register_decode_chain+0x18/0x20 > [ 21.790430] [] i7core_register_mci+0x2fb/0x3e4 [i7core_edac] > [ 21.798003] [] i7core_probe+0xd4/0x5c0 [i7core_edac] > [ 21.804809] [] local_pci_probe+0x5b/0xe0 > [ 21.810631] [] __pci_device_probe+0xd9/0xe0 > [ 21.816650] [] ? get_device+0x14/0x20 > [ 21.822178] [] pci_device_probe+0x36/0x60 > [ 21.828061] [] really_probe+0x7a/0x2b0 > [ 21.833676] [] driver_probe_device+0x63/0xc0 > [ 21.839868] [] __driver_attach+0x9b/0xa0 > [ 21.845718] [] ? driver_probe_device+0xc0/0xc0 > [ 21.852027] [] bus_for_each_dev+0x68/0x90 > [ 21.857876] [] driver_attach+0x1c/0x20 > [ 21.863462] [] bus_add_driver+0x16d/0x2b0 > [ 21.869377] [] driver_register+0x7c/0x160 > [ 21.875220] [] __pci_register_driver+0x6a/0xf0 > [ 21.881494] [] ? 0xffffffffa01fdfff > [ 21.886846] [] i7core_init+0x47/0x1000 [i7core_edac] > [ 21.893737] [] do_one_initcall+0x3e/0x180 > [ 21.899670] [] sys_init_module+0xc5/0x220 > [ 21.905542] [] system_call_fastpath+0x16/0x1b > > Fix this by using rcu_access_index() instead of rcu_dereference_check_mce(). > This is similar to what was done in commit a4dd992 (rcu: create new > rcu_access_index() and use in mce) for a similar case in mce_poll. > Link to that discussion: http://thread.gmane.org/gmane.linux.kernel/1058460/ > > Signed-off-by: Srivatsa S. Bhat > --- > I am no MCE expert. Hence requesting a thorough review of the patch. > > arch/x86/kernel/cpu/mcheck/mce.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c > index f22a9f7..c191fec 100644 > --- a/arch/x86/kernel/cpu/mcheck/mce.c > +++ b/arch/x86/kernel/cpu/mcheck/mce.c > @@ -191,7 +191,7 @@ static void drain_mcelog_buffer(void) > { > unsigned int next, i, prev = 0; > > - next = rcu_dereference_check_mce(mcelog.next); > + next = rcu_access_index(mcelog.next); So this is not so much a use of RCU as it is a standard mailbox algorithm, with the ->finished flag controlling access. So I suggest ACCESS_ONCE() for the accesses to mcelog.next. Thanx, Paul > do { > struct mce *m; >