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Date: Mon, 16 Aug 2010 15:57:47 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Miles Lane <miles.lane@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, dhowells@redhat.com
Subject: Re: 2.6.35 - INFO: kernel/exit.c:1387 invoked
 rcu_dereference_check() without protection!
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On Mon, Aug 16, 2010 at 03:37:30PM -0700, Miles Lane wrote:
> Hi Paul and friends,
> 
> Now in 2.6.36-rc1:

Hello, Miles!

Could you please try David Howells's patch?  It may be found at:

	http://lkml.org/lkml/2010/8/16/333

It should address this one.

							Thanx, Paul

> [    7.295797] ===================================================
> [    7.295801] [ INFO: suspicious rcu_dereference_check() usage. ]
> [    7.295805] ---------------------------------------------------
> [    7.295810] kernel/exit.c:1390 invoked rcu_dereference_check()
> without protection!
> [    7.295813]
> [    7.295814] other info that might help us debug this:
> [    7.295816]
> [    7.295819]
> [    7.295820] rcu_scheduler_active = 1, debug_locks = 1
> [    7.295825] 2 locks held by init/1:
> [    7.295827]  #0:  (tasklist_lock){.+.+..}, at: [<ffffffff8103e31c>]
> do_wait+0xa5/0x1fa
> [    7.295843]  #1:  (&(&sighand->siglock)->rlock){......}, at:
> [<ffffffff8103de60>] wait_consider_task+0x5e1/0x9f8
> [    7.295854]
> [    7.295855] stack backtrace:
> [    7.295860] Pid: 1, comm: init Not tainted 2.6.36-rc1 #3
> [    7.295864] Call Trace:
> [    7.295872]  [<ffffffff81061999>] lockdep_rcu_dereference+0x9d/0xa6
> [    7.295878]  [<ffffffff8103deef>] wait_consider_task+0x670/0x9f8
> [    7.295884]  [<ffffffff8103e388>] do_wait+0x111/0x1fa
> [    7.295890]  [<ffffffff8103e5b9>] sys_waitid+0x7f/0x178
> [    7.295898]  [<ffffffff81002a5c>] ? sysret_check+0x27/0x62
> [    7.295904]  [<ffffffff8103cbc6>] ? child_wait_callback+0x0/0x53
> [    7.295911]  [<ffffffff81002a2b>] system_call_fastpath+0x16/0x1b
> 
> 
> On Mon, Aug 9, 2010 at 10:27 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Aug 03, 2010 at 11:20:58PM -0400, Miles Lane wrote:
> >> [ INFO: suspicious rcu_dereference_check() usage. ]
> >> ---------------------------------------------------
> >> kernel/exit.c:1387 invoked rcu_dereference_check() without protection!
> >>
> >> other info that might help us debug this:
> >>
> >> rcu_scheduler_active = 1, debug_locks = 1
> >> 2 locks held by init/1:
> >> ?#0: ?(tasklist_lock){.+.+..}, at: [<ffffffff81045ca8>] do_wait+0xa9/0x1fa
> >> ?#1: ?(&(&sighand->siglock)->rlock){......}, at: [<ffffffff810457e8>]
> >> wait_consider_task+0x5e1/0x9f8
> >>
> >> stack backtrace:
> >> Pid: 1, comm: init Not tainted 2.6.35 #15
> >> Call Trace:
> >> ?[<ffffffff8106759c>] lockdep_rcu_dereference+0x9d/0xa6
> >> ?[<ffffffff81045877>] wait_consider_task+0x670/0x9f8
> >> ?[<ffffffff81045d14>] do_wait+0x115/0x1fa
> >> ?[<ffffffff81045f41>] sys_waitid+0x7f/0x178
> >> ?[<ffffffff81009cba>] ? sysret_check+0x2e/0x69
> >> ?[<ffffffff8104454e>] ? child_wait_callback+0x0/0x53
> >> ?[<ffffffff81009c82>] system_call_fastpath+0x16/0x1b
> >
> > This one is interesting. ?The ->sighand->siglock is held, but the
> > rcu_dereference_check() check condition requires that either the
> > task is dead or that we are in an RCU read-side critical section.
> > The comment preceding the call to __task_cred() claims that we
> > "don't need the RCU readlock here as we're holding a spinlock."
> > This comment dates back to 2008, so might be obsolete.
> >
> > David, should we enclose the __task_cred() in wait_task_stopped()
> > with rcu_read_lock()? ?Or would it be better to add a check to
> > __task_cred() checking for ->sighand->siglock? ?Or do we need to
> > do something else entirely? ?;-)
> >
> > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Thanx, Paul
> >
> --
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