From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: 2.6.34-rc5-git7 (plus all patches) -- another suspicious rcu_dereference_check() usage. Date: Wed, 28 Apr 2010 13:09:04 -0700 Message-ID: <20100428200904.GS2540@linux.vnet.ibm.com> References: <20100428175426.GK2540@linux.vnet.ibm.com> <1272483491.2201.9.camel@edumazet-laptop> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 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: Eric Dumazet Return-path: Content-Disposition: inline In-Reply-To: <1272483491.2201.9.camel@edumazet-laptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Apr 28, 2010 at 09:38:11PM +0200, Eric Dumazet wrote: > Le mercredi 28 avril 2010 =E0 10:54 -0700, Paul E. McKenney a =E9crit= : > > 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-rc= 5-git7 #33 > > > [ 984.724724] Call Trace: > > > [ 984.724734] [] lockdep_rcu_dereference+0x9d= /0xa6 > > > [ 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+0x16/0x= 1b > >=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) pa= ssing > > in a files_struct with ->count equal to 1 (initialization or cleanu= p). > >=20 > > So I don't feel comfortable just slapping an RCU read-side critical > > section around this one, at least not unless someone who understand= s > > the locking says that doing so is OK. > >=20 > > =09 >=20 > Its a single threaded program. >=20 > 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. So I should add a single-threaded check. My first thought was to use current_is_single_threaded(), but the bit about scanning the full list of processes does give me pause. However, thread_group_empty() looks like a much lighter-weight alternative. I believe that it is possible for a pair of single-threaded processes to share a file descriptor, but that should not be a problem, as both of them would need to close it for it to go away. But what happens if someone does a clone() with CLONE_FILES, as some of the AIO stuff seems to do? Won't that allow one of the resulting processes to close the file for both of them, even though both are otherwise single-threaded? And the ->count seems to be the only distinction between these two cases. And AIO does CLONE_VM as well as CLONE_FILES, but that seems to mean th= at the check must scan the processes with current_is_single_threaded(). Besides which, a user could invoke clone() with only CLONE_FILES specified, right? Or am I just confused here? Thanx, Paul