From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: 2.6.34-rc5-git7 (plus all patches) -- another suspicious rcu_dereference_check() usage. Date: Wed, 28 Apr 2010 21:38:11 +0200 Message-ID: <1272483491.2201.9.camel@edumazet-laptop> References: <20100428175426.GK2540@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Miles Lane , Vivek Goyal , Eric Paris , Lai Jiangshan , Ingo Molnar , Peter Zijlstra , LKML , nauman@google.com, netdev@vger.kernel.org, Jens Axboe , Gui Jianfeng , Li Zefan , Johannes Berg , shemminger@vyatta.com To: paulmck@linux.vnet.ibm.com Return-path: In-Reply-To: <20100428175426.GK2540@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Le mercredi 28 avril 2010 =C3=A0 10:54 -0700, Paul E. McKenney a =C3=A9= crit : > On Mon, Apr 26, 2010 at 08:51:06PM -0400, Miles Lane wrote: > > This one occurred during the wakeup from suspend to RAM. > >=20 > > [ 984.724697] [ INFO: suspicious rcu_dereference_check() usage. ] > > [ 984.724700] --------------------------------------------------- > > [ 984.724703] include/linux/fdtable.h:88 invoked > > rcu_dereference_check() without protection! > > [ 984.724706] > > [ 984.724707] other info that might help us debug this: > > [ 984.724708] > > [ 984.724711] > > [ 984.724711] rcu_scheduler_active =3D 1, debug_locks =3D 1 > > [ 984.724714] no locks held by dbus-daemon/4680. > > [ 984.724717] > > [ 984.724717] stack backtrace: > > [ 984.724721] Pid: 4680, comm: dbus-daemon Not tainted 2.6.34-rc5-= git7 #33 > > [ 984.724724] Call Trace: > > [ 984.724734] [] lockdep_rcu_dereference+0x9d/0= xa6 > > [ 984.724740] [] fcheck_files+0xb1/0xc9 > > [ 984.724745] [] fget_light+0x35/0xab > > [ 984.724751] [] ? sock_poll_wait+0x13/0x18 > > [ 984.724755] [] ? unix_poll+0x19/0x95 > > [ 984.724762] [] do_sys_poll+0x1ff/0x3e5 > > [ 984.724766] [] ? __pollwait+0x0/0xc7 > > [ 984.724771] [] ? pollwake+0x0/0x4f > > [ 984.724776] [] ? pollwake+0x0/0x4f > > [ 984.724780] [] ? pollwake+0x0/0x4f > > [ 984.724784] [] ? pollwake+0x0/0x4f > > [ 984.724788] [] ? pollwake+0x0/0x4f > > [ 984.724793] [] ? pollwake+0x0/0x4f > > [ 984.724797] [] ? pollwake+0x0/0x4f > > [ 984.724802] [] ? pollwake+0x0/0x4f > > [ 984.724806] [] ? pollwake+0x0/0x4f > > [ 984.724812] [] sys_poll+0x50/0xbb > > [ 984.724818] [] system_call_fastpath+0x16/0x1b >=20 > Hmmm... I am not convinced that this is a false positive. Couldn't > there be a multi-threaded process where one thread is invoking poll() > on a UNIX socket just as another thread is calling close() on it? >=20 > The current fcheck_files() logic requires that the caller either (1) = be in > an RCU read-side critical section, (2) hold ->files_lock, or (3) pass= ing > in a files_struct with ->count equal to 1 (initialization or cleanup)= =2E >=20 > So I don't feel comfortable just slapping an RCU read-side critical > section around this one, at least not unless someone who understands > the locking says that doing so is OK. >=20 > =09 Its a single threaded program. So fget_light() calls fcheck_files(files, fd); without rcu lock, but some /proc/pid/fd/... user temporarly raised files->count just before we perform the condition check.