New RCU related warning due to rcu_preempt_depth() changes

New RCU related warning due to rcu_preempt_depth() changes

[Sasha Levin]

Hi Paul,

It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
__rcu_read_unlock() to per-CPU variables") is causing the following
warning (I've added the extra fields on the second line):

[   77.330920] BUG: sleeping function called from invalid context at
mm/memory.c:3933
[   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
[   77.344135] no locks held by trinity/5669.
[   77.349644] Pid: 5669, comm: trinity Tainted: G        W
3.4.0-rc3-next-20120417-sasha-dirty #83
[   77.354401] Call Trace:
[   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
[   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
[   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
[   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80

As you can see, rcu_preempt_depth() returns 1 when running in that
context, which looks pretty odd.

Thanks.

Re: New RCU related warning due to rcu_preempt_depth() changes

[Paul E. McKenney]

On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
> Hi Paul,
> 
> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
> __rcu_read_unlock() to per-CPU variables") is causing the following
> warning (I've added the extra fields on the second line):
> 
> [   77.330920] BUG: sleeping function called from invalid context at
> mm/memory.c:3933
> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
> [   77.344135] no locks held by trinity/5669.
> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
> 3.4.0-rc3-next-20120417-sasha-dirty #83
> [   77.354401] Call Trace:
> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
> 
> As you can see, rcu_preempt_depth() returns 1 when running in that
> context, which looks pretty odd.

Ouch!!!

So it looks like I missed a place where I need to save and restore
the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
variables.  My (probably hopelessly naive) guess is that I need to add
a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
make rcu_switch_from() take the task_struct pointer as an argument,
passing in prev.

Does this make sense, or am I still missing something here?

							Thanx, Paul

Re: New RCU related warning due to rcu_preempt_depth() changes

[Sasha Levin]

On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
>> Hi Paul,
>>
>> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
>> __rcu_read_unlock() to per-CPU variables") is causing the following
>> warning (I've added the extra fields on the second line):
>>
>> [   77.330920] BUG: sleeping function called from invalid context at
>> mm/memory.c:3933
>> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
>> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
>> [   77.344135] no locks held by trinity/5669.
>> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
>> 3.4.0-rc3-next-20120417-sasha-dirty #83
>> [   77.354401] Call Trace:
>> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
>> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
>> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
>> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
>>
>> As you can see, rcu_preempt_depth() returns 1 when running in that
>> context, which looks pretty odd.
>
> Ouch!!!
>
> So it looks like I missed a place where I need to save and restore
> the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
> variables.  My (probably hopelessly naive) guess is that I need to add
> a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
> make rcu_switch_from() take the task_struct pointer as an argument,
> passing in prev.
>
> Does this make sense, or am I still missing something here?

I've let the test run for a bit more, and it appears that I'm getting
this warning from lots of different sources, would this
schedule_tail() fix all of them?

Here's several traces for reference:

[  223.068875]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
[  223.070719]  [<ffffffff810b2a05>] close_files+0x1d5/0x220
[  223.072531]  [<ffffffff810b2830>] ? find_new_reaper+0x230/0x230
[  223.076325]  [<ffffffff810b4811>] put_files_struct+0x21/0x1b0
[  223.080649]  [<ffffffff8269eb20>] ? _raw_spin_unlock+0x30/0x60
[  223.084455]  [<ffffffff810b4a5d>] exit_files+0x4d/0x60
[  223.087967]  [<ffffffff810b51ac>] do_exit+0x28c/0x470
[  223.091369]  [<ffffffff810e44d1>] ? get_parent_ip+0x11/0x50
[  223.093190]  [<ffffffff810b5473>] do_group_exit+0xa3/0xe0
[  223.095061]  [<ffffffff810c4759>] get_signal_to_deliver+0x389/0x400
[  223.098400]  [<ffffffff8104bce2>] do_signal+0x42/0x120
[  223.100222]  [<ffffffff8104c4d7>] ? do_divide_error+0xa7/0xb0
[  223.102267]  [<ffffffff8269fa7f>] ? retint_signal+0x11/0x92
[  223.104145]  [<ffffffff8104be34>] do_notify_resume+0x54/0xa0
[  223.106033]  [<ffffffff8269fabb>] retint_signal+0x4d/0x92

[  176.217632]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
[  176.223583]  [<ffffffff81081b83>] do_page_fault+0x243/0x4f0
[  176.229932]  [<ffffffff811151ca>] ? __lock_release+0x1ba/0x1d0
[  176.233651]  [<ffffffff8269ec1b>] ? _raw_spin_unlock_irq+0x2b/0x80
[  176.239389]  [<ffffffff810e44d1>] ? get_parent_ip+0x11/0x50
[  176.242507]  [<ffffffff810e469e>] ? sub_preempt_count+0xae/0xe0
[  176.248795]  [<ffffffff8269ec41>] ? _raw_spin_unlock_irq+0x51/0x80
[  176.255454]  [<ffffffff81079e51>] do_async_page_fault+0x31/0xa0
[  176.260342]  [<ffffffff8269fcd5>] async_page_fault+0x25/0x30

[  173.587864]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
[  173.593134]  [<ffffffff811f5542>] ? __d_alloc+0x32/0x1a0
[  173.603730]  [<ffffffff811c0f8d>] kmem_cache_alloc+0x4d/0x160
[  173.604746]  [<ffffffff811f5e40>] ? __d_lookup_rcu+0x240/0x240
[  173.608932]  [<ffffffff811f5542>] __d_alloc+0x32/0x1a0
[  173.612444]  [<ffffffff811f56f3>] d_alloc+0x23/0x80
[  173.616887]  [<ffffffff811e773b>] __lookup_hash+0x9b/0x110
[  173.621488]  [<ffffffff811e77c4>] lookup_hash+0x14/0x20
[  173.624395]  [<ffffffff811ecc99>] do_unlinkat+0x79/0x1e0
[  173.626483]  [<ffffffff8269ec41>] ? _raw_spin_unlock_irq+0x51/0x80
[  173.632242]  [<ffffffff826a02e9>] ? sysret_check+0x22/0x5d
[  173.637884]  [<ffffffff8186db5e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[  173.642176]  [<ffffffff811ece51>] sys_unlink+0x11/0x20
[  173.645320]  [<ffffffff826a02bd>] system_call_fastpath+0x1a/0x1f

Re: New RCU related warning due to rcu_preempt_depth() changes

[Paul E. McKenney]

On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote:
> On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
> >> Hi Paul,
> >>
> >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
> >> __rcu_read_unlock() to per-CPU variables") is causing the following
> >> warning (I've added the extra fields on the second line):
> >>
> >> [   77.330920] BUG: sleeping function called from invalid context at
> >> mm/memory.c:3933
> >> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
> >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
> >> [   77.344135] no locks held by trinity/5669.
> >> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
> >> 3.4.0-rc3-next-20120417-sasha-dirty #83
> >> [   77.354401] Call Trace:
> >> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> >> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
> >> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
> >> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
> >>
> >> As you can see, rcu_preempt_depth() returns 1 when running in that
> >> context, which looks pretty odd.
> >
> > Ouch!!!
> >
> > So it looks like I missed a place where I need to save and restore
> > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
> > variables.  My (probably hopelessly naive) guess is that I need to add
> > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
> > make rcu_switch_from() take the task_struct pointer as an argument,
> > passing in prev.
> >
> > Does this make sense, or am I still missing something here?
> 
> I've let the test run for a bit more, and it appears that I'm getting
> this warning from lots of different sources, would this
> schedule_tail() fix all of them?

If I understand the failure correctly, yes.  If the task switches without
RCU paying attention, the nesting count for both the outgoing and the
incoming tasks can get messed up.  The messed-up counts could easily
cause problems downstream.

Of course, there might well be additional bugs.

I will put a speculative patch together and send it along.

							Thanx, Paul

> Here's several traces for reference:
> 
> [  223.068875]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> [  223.070719]  [<ffffffff810b2a05>] close_files+0x1d5/0x220
> [  223.072531]  [<ffffffff810b2830>] ? find_new_reaper+0x230/0x230
> [  223.076325]  [<ffffffff810b4811>] put_files_struct+0x21/0x1b0
> [  223.080649]  [<ffffffff8269eb20>] ? _raw_spin_unlock+0x30/0x60
> [  223.084455]  [<ffffffff810b4a5d>] exit_files+0x4d/0x60
> [  223.087967]  [<ffffffff810b51ac>] do_exit+0x28c/0x470
> [  223.091369]  [<ffffffff810e44d1>] ? get_parent_ip+0x11/0x50
> [  223.093190]  [<ffffffff810b5473>] do_group_exit+0xa3/0xe0
> [  223.095061]  [<ffffffff810c4759>] get_signal_to_deliver+0x389/0x400
> [  223.098400]  [<ffffffff8104bce2>] do_signal+0x42/0x120
> [  223.100222]  [<ffffffff8104c4d7>] ? do_divide_error+0xa7/0xb0
> [  223.102267]  [<ffffffff8269fa7f>] ? retint_signal+0x11/0x92
> [  223.104145]  [<ffffffff8104be34>] do_notify_resume+0x54/0xa0
> [  223.106033]  [<ffffffff8269fabb>] retint_signal+0x4d/0x92
> 
> [  176.217632]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> [  176.223583]  [<ffffffff81081b83>] do_page_fault+0x243/0x4f0
> [  176.229932]  [<ffffffff811151ca>] ? __lock_release+0x1ba/0x1d0
> [  176.233651]  [<ffffffff8269ec1b>] ? _raw_spin_unlock_irq+0x2b/0x80
> [  176.239389]  [<ffffffff810e44d1>] ? get_parent_ip+0x11/0x50
> [  176.242507]  [<ffffffff810e469e>] ? sub_preempt_count+0xae/0xe0
> [  176.248795]  [<ffffffff8269ec41>] ? _raw_spin_unlock_irq+0x51/0x80
> [  176.255454]  [<ffffffff81079e51>] do_async_page_fault+0x31/0xa0
> [  176.260342]  [<ffffffff8269fcd5>] async_page_fault+0x25/0x30
> 
> [  173.587864]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> [  173.593134]  [<ffffffff811f5542>] ? __d_alloc+0x32/0x1a0
> [  173.603730]  [<ffffffff811c0f8d>] kmem_cache_alloc+0x4d/0x160
> [  173.604746]  [<ffffffff811f5e40>] ? __d_lookup_rcu+0x240/0x240
> [  173.608932]  [<ffffffff811f5542>] __d_alloc+0x32/0x1a0
> [  173.612444]  [<ffffffff811f56f3>] d_alloc+0x23/0x80
> [  173.616887]  [<ffffffff811e773b>] __lookup_hash+0x9b/0x110
> [  173.621488]  [<ffffffff811e77c4>] lookup_hash+0x14/0x20
> [  173.624395]  [<ffffffff811ecc99>] do_unlinkat+0x79/0x1e0
> [  173.626483]  [<ffffffff8269ec41>] ? _raw_spin_unlock_irq+0x51/0x80
> [  173.632242]  [<ffffffff826a02e9>] ? sysret_check+0x22/0x5d
> [  173.637884]  [<ffffffff8186db5e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> [  173.642176]  [<ffffffff811ece51>] sys_unlink+0x11/0x20
> [  173.645320]  [<ffffffff826a02bd>] system_call_fastpath+0x1a/0x1f
> 

Re: New RCU related warning due to rcu_preempt_depth() changes

[Paul E. McKenney]

On Tue, Apr 17, 2012 at 08:53:16AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote:
> > On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney
> > <paulmck@linux.vnet.ibm.com> wrote:
> > > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
> > >> Hi Paul,
> > >>
> > >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
> > >> __rcu_read_unlock() to per-CPU variables") is causing the following
> > >> warning (I've added the extra fields on the second line):
> > >>
> > >> [   77.330920] BUG: sleeping function called from invalid context at
> > >> mm/memory.c:3933
> > >> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
> > >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
> > >> [   77.344135] no locks held by trinity/5669.
> > >> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
> > >> 3.4.0-rc3-next-20120417-sasha-dirty #83
> > >> [   77.354401] Call Trace:
> > >> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> > >> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
> > >> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
> > >> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
> > >>
> > >> As you can see, rcu_preempt_depth() returns 1 when running in that
> > >> context, which looks pretty odd.
> > >
> > > Ouch!!!
> > >
> > > So it looks like I missed a place where I need to save and restore
> > > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
> > > variables.  My (probably hopelessly naive) guess is that I need to add
> > > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
> > > make rcu_switch_from() take the task_struct pointer as an argument,
> > > passing in prev.
> > >
> > > Does this make sense, or am I still missing something here?
> > 
> > I've let the test run for a bit more, and it appears that I'm getting
> > this warning from lots of different sources, would this
> > schedule_tail() fix all of them?
> 
> If I understand the failure correctly, yes.  If the task switches without
> RCU paying attention, the nesting count for both the outgoing and the
> incoming tasks can get messed up.  The messed-up counts could easily
> cause problems downstream.
> 
> Of course, there might well be additional bugs.
> 
> I will put a speculative patch together and send it along.

And here it is, testing just started.

							Thanx, Paul

------------------------------------------------------------------------

rcu: Add RCU context switching to schedule_tail()

The new rcu_read_lock_nesting and rcu_read_unlock_special per-CPU
variables must be saved and restored at every context switch, including
those involving schedule_tail().  This commit therefore adds the saving
and restoring to schedul_tail().

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index 25a7fea..32272d4 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -704,7 +704,7 @@ static void stack_proc(void *arg)
 	struct task_struct *from = current, *to = arg;
 
 	to->thread.saved_task = from;
-	rcu_switch_from();
+	rcu_switch_from(from);
 	switch_to(from, to, from);
 	rcu_switch_to();
 }
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f2468cb..0d48609 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1937,10 +1937,8 @@ static inline void rcu_copy_process(struct task_struct *p)
  *
  * The caller must have disabled preemption.
  */
-static inline void rcu_switch_from(void)
+static inline void rcu_switch_from(struct task_struct *t)
 {
-	struct task_struct *t = current;
-
 	if (__this_cpu_read(rcu_read_lock_nesting) != 0)
 		rcu_preempt_note_context_switch();
 	t->rcu_read_lock_nesting_save = __this_cpu_read(rcu_read_lock_nesting);
@@ -1991,7 +1989,7 @@ static inline void rcu_copy_process(struct task_struct *p)
 {
 }
 
-static inline void rcu_switch_from(void)
+static inline void rcu_switch_from(struct task_struct *t)
 {
 }
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 51ce537..17ae267 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2024,6 +2024,8 @@ asmlinkage void schedule_tail(struct task_struct *prev)
 {
 	struct rq *rq = this_rq();
 
+	rcu_switch_from(prev);
+	rcu_switch_to();
 	finish_task_switch(rq, prev);
 
 	/*
@@ -2083,7 +2085,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 #endif
 
 	/* Here we just switch the register state and the stack. */
-	rcu_switch_from();
+	rcu_switch_from(current);
 	switch_to(prev, next, prev);
 	rcu_switch_to();
 

Re: New RCU related warning due to rcu_preempt_depth() changes

[Sasha Levin]

On Tue, Apr 17, 2012 at 6:45 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Tue, Apr 17, 2012 at 08:53:16AM -0700, Paul E. McKenney wrote:
>> On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote:
>> > On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney
>> > <paulmck@linux.vnet.ibm.com> wrote:
>> > > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
>> > >> Hi Paul,
>> > >>
>> > >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
>> > >> __rcu_read_unlock() to per-CPU variables") is causing the following
>> > >> warning (I've added the extra fields on the second line):
>> > >>
>> > >> [   77.330920] BUG: sleeping function called from invalid context at
>> > >> mm/memory.c:3933
>> > >> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
>> > >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
>> > >> [   77.344135] no locks held by trinity/5669.
>> > >> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
>> > >> 3.4.0-rc3-next-20120417-sasha-dirty #83
>> > >> [   77.354401] Call Trace:
>> > >> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
>> > >> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
>> > >> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
>> > >> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
>> > >>
>> > >> As you can see, rcu_preempt_depth() returns 1 when running in that
>> > >> context, which looks pretty odd.
>> > >
>> > > Ouch!!!
>> > >
>> > > So it looks like I missed a place where I need to save and restore
>> > > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
>> > > variables.  My (probably hopelessly naive) guess is that I need to add
>> > > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
>> > > make rcu_switch_from() take the task_struct pointer as an argument,
>> > > passing in prev.
>> > >
>> > > Does this make sense, or am I still missing something here?
>> >
>> > I've let the test run for a bit more, and it appears that I'm getting
>> > this warning from lots of different sources, would this
>> > schedule_tail() fix all of them?
>>
>> If I understand the failure correctly, yes.  If the task switches without
>> RCU paying attention, the nesting count for both the outgoing and the
>> incoming tasks can get messed up.  The messed-up counts could easily
>> cause problems downstream.
>>
>> Of course, there might well be additional bugs.
>>
>> I will put a speculative patch together and send it along.
>
> And here it is, testing just started.
>
>                                                        Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Add RCU context switching to schedule_tail()
>
> The new rcu_read_lock_nesting and rcu_read_unlock_special per-CPU
> variables must be saved and restored at every context switch, including
> those involving schedule_tail().  This commit therefore adds the saving
> and restoring to schedul_tail().
>
> Reported-by: Sasha Levin <levinsasha928@gmail.com>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>

Looks good here.

Thanks.

Re: New RCU related warning due to rcu_preempt_depth() changes

[Paul E. McKenney]

On Wed, Apr 18, 2012 at 07:29:19AM +0200, Sasha Levin wrote:
> On Tue, Apr 17, 2012 at 6:45 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Apr 17, 2012 at 08:53:16AM -0700, Paul E. McKenney wrote:
> >> On Tue, Apr 17, 2012 at 05:36:59PM +0200, Sasha Levin wrote:
> >> > On Tue, Apr 17, 2012 at 5:05 PM, Paul E. McKenney
> >> > <paulmck@linux.vnet.ibm.com> wrote:
> >> > > On Tue, Apr 17, 2012 at 10:42:47AM +0200, Sasha Levin wrote:
> >> > >> Hi Paul,
> >> > >>
> >> > >> It looks like commit 7298b03 ("rcu: Move __rcu_read_lock() and
> >> > >> __rcu_read_unlock() to per-CPU variables") is causing the following
> >> > >> warning (I've added the extra fields on the second line):
> >> > >>
> >> > >> [   77.330920] BUG: sleeping function called from invalid context at
> >> > >> mm/memory.c:3933
> >> > >> [   77.336571] in_atomic(): 0, irqs_disabled(): 0, preempt count: 0,
> >> > >> preempt offset: 0, rcu depth: 1, pid: 5669, name: trinity
> >> > >> [   77.344135] no locks held by trinity/5669.
> >> > >> [   77.349644] Pid: 5669, comm: trinity Tainted: G        W
> >> > >> 3.4.0-rc3-next-20120417-sasha-dirty #83
> >> > >> [   77.354401] Call Trace:
> >> > >> [   77.355956]  [<ffffffff810e83f3>] __might_sleep+0x1f3/0x210
> >> > >> [   77.358811]  [<ffffffff81198eaf>] might_fault+0x2f/0xa0
> >> > >> [   77.361997]  [<ffffffff810e3228>] schedule_tail+0x88/0xb0
> >> > >> [   77.364671]  [<ffffffff826a01d3>] ret_from_fork+0x13/0x80
> >> > >>
> >> > >> As you can see, rcu_preempt_depth() returns 1 when running in that
> >> > >> context, which looks pretty odd.
> >> > >
> >> > > Ouch!!!
> >> > >
> >> > > So it looks like I missed a place where I need to save and restore
> >> > > the new per-CPU rcu_read_lock_nesting and rcu_read_unlock_special
> >> > > variables.  My (probably hopelessly naive) guess is that I need to add
> >> > > a rcu_switch_from() and rcu_switch_to() into schedule_tail(), but to
> >> > > make rcu_switch_from() take the task_struct pointer as an argument,
> >> > > passing in prev.
> >> > >
> >> > > Does this make sense, or am I still missing something here?
> >> >
> >> > I've let the test run for a bit more, and it appears that I'm getting
> >> > this warning from lots of different sources, would this
> >> > schedule_tail() fix all of them?
> >>
> >> If I understand the failure correctly, yes.  If the task switches without
> >> RCU paying attention, the nesting count for both the outgoing and the
> >> incoming tasks can get messed up.  The messed-up counts could easily
> >> cause problems downstream.
> >>
> >> Of course, there might well be additional bugs.
> >>
> >> I will put a speculative patch together and send it along.
> >
> > And here it is, testing just started.
> >
> >                                                        Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > rcu: Add RCU context switching to schedule_tail()
> >
> > The new rcu_read_lock_nesting and rcu_read_unlock_special per-CPU
> > variables must be saved and restored at every context switch, including
> > those involving schedule_tail().  This commit therefore adds the saving
> > and restoring to schedul_tail().
> >
> > Reported-by: Sasha Levin <levinsasha928@gmail.com>
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Looks good here.

Very good!  I have added your Tested-by.

							Thanx, Paul