Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Nov 27, 2012 at 04:39:59PM +0100, Frederic Weisbecker wrote:
> 2012/11/27 Li Zhong <zhong@linux.vnet.ibm.com>:
> > I noticed some warnings complaining about dynticks_nesting value, like
> >
> > [  267.545032] ------------[ cut here ]------------
> > [  267.545032] WARNING: at kernel/rcutree.c:382 rcu_eqs_enter+0xab/0xc0()
> > [  267.545032] Hardware name: Bochs
> > [  267.545032] Modules linked in:
> > [  267.545032] Pid: 0, comm: swapper/2 Not tainted 3.7.0-rc5-next-20121115 #8
> > [  267.545032] Call Trace:
> > [  267.545032]  [<ffffffff8104714f>] warn_slowpath_common+0x7f/0xc0
> > [  267.545032]  [<ffffffff810471aa>] warn_slowpath_null+0x1a/0x20
> > [  267.545032]  [<ffffffff810e607b>] rcu_eqs_enter+0xab/0xc0
> > [  267.545032]  [<ffffffff810e60bb>] rcu_idle_enter+0x2b/0x70
> > [  267.545032]  [<ffffffff8100d44f>] cpu_idle+0x6f/0x100
> > [  267.545032]  [<ffffffff814bf055>] start_secondary+0x205/0x20c
> > [  267.545032] ---[ end trace 924ae80da035028d ]---
> >
> > After enabling rcu-dyntick tracing, I got following abnormal
> > dynticks_nesting values (13fffffffffffff, ff00000000000001,etc):
> >                         ...
> >  1      <idle>-0     [002] dN.2 18739.518567: rcu_dyntick: End 0 140000000000000                rcu_idle_exit
> >  2        sshd-696   [002] d..1 18739.518675: rcu_dyntick: ++= 140000000000000 140000000000001  rcu_irq_enter   - apf (not present)
> >
> >  3      <idle>-0     [002] d..2 18739.518705: rcu_dyntick: Start 140000000000001 0              rcu_idle_enter
> >  4      <idle>-0     [002] d..2 18739.521252: rcu_dyntick: End 0 1                              rcu_irq_enter   - apf (page ready)
> >  5      <idle>-0     [002] dN.2 18739.521261: rcu_dyntick: Start 1 0                            rcu_irq_exit    - apf (page ready)
> >  6      <idle>-0     [002] dN.2 18739.521263: rcu_dyntick: End 0 140000000000000                rcu_idle_exit
> >
> >  7        sshd-696   [002] d..1 18739.521299: rcu_dyntick: --= 140000000000000 13fffffffffffff  rcu_irq_exit    - apf (not present)
> 
> Calling rcu_irq_exit() without a matching rcu_irq_enter() after the
> last rcu_idle_exit() is illegal, isn't it?

It is OK to call rcu_irq_exit() without a matching rcu_irq_enter() -only-
if you have also called rcu_idle_exit() since the last rcu_idle_enter().
There will be a similar rule for rcu_user_exit().

More generally, it is OK to call rcu_irq_exit() without a matching
rcu_irq_enter() only if RCU believes that the CPU you are running on is
non-idle.  On 32-bit systems, you are only allowed a few tens of million
such unmatched rcu_irq_enter() calls in a given RCU-non-idle region.

All courtesy of RCU's need to tolerate architectures that enter
interrupt handlers without ever leaving them and vice versa.  ;-)

							Thanx, Paul