Re: [PATCH -rt] drop spurious rcu unlock

[Paul E. McKenney]

On Mon, Jul 23, 2007 at 06:37:19AM -0700, Daniel Walker wrote:
> On Sun, 2007-07-22 at 20:13 -0700, Paul E. McKenney wrote:
> > On Sun, Jul 22, 2007 at 10:22:37AM -0700, Daniel Walker wrote:
> > > 
> > > Strange rcu_read_unlock() which causes a imbalance, and boot hang.. I
> > > didn't notice a reason for it, and removing it allows my system to make
> > > progress.
> > > 
> > > This should go into the preempt-realtime-sched.patch
> > 
> > Strange.  I have been getting boots, kernbench runs, and rcutorture
> > tests despite its being present.  Though perhaps it was the cause
> > of the (non-fatal) "scheduling while atomic" I got during a kernbench
> > run -- I was assuming it was my fault somehow.  Non-reproducible.  :-/
> > 
> > 						Thanx, Paul
> 
> I had a strange config (for real time) .. All the new options turned
> off, and classic RCU .. However, a regular PREEMPT_RT kernel booted
> fine. The classic RCU made it easy to spot this since the preempt count
> goes negative.

Ah!  That would happen in a CONFIG_PREEMPT kernel.  The bug would be
invisible in a non-CONFIG_PREEMPT kernel.

> I was wondering what kind of side effects would happen with the
> preemptible RCU ..

Well, if you had only one rcu_read_lock() outstanding in the system,
it would appear to be in a quiescent state, which would not be good.
Given enough executions of that code, grace periods would cease and
you would OOM.

I didn't see any OOMs despite overnight rcutorture runs -- which would
definitely have spotted halted grace periods -- so perhaps I just happened
to never invoke sched_yield?

> > BUG: sleeping function called from invalid context cc1(29651) at kernel/rtmutex.c:636
> > in_atomic():1 [00000001], irqs_disabled():0
> >  [<c0119f50>] __might_sleep+0xf3/0xf9
> >  [<c031600e>] __rt_spin_lock+0x21/0x3c
> >  [<c014102c>] get_zone_pcp+0x20/0x29
> >  [<c0141a40>] free_hot_cold_page+0xdc/0x167
> >  [<c013a3f4>] add_preempt_count+0x12/0xcc
> >  [<c0110d92>] pgd_dtor+0x0/0x1
> >  [<c015d865>] quicklist_trim+0xb7/0xe3
> >  [<c0111025>] check_pgt_cache+0x19/0x1c
> >  [<c0148df5>] free_pgtables+0x54/0x12c
> >  [<c013a3f4>] add_preempt_count+0x12/0xcc
> >  [<c014e5be>] unmap_region+0xeb/0x13b
> 
> This looks a little like what Rui Nuno Capela is seeing , but his kernel
> hangs.

Hmmm...  In this particular case, the test (kernbench) was fairly
short, so maybe I just didn't let it run long enough.

						Thanx, Paul

> > BUG: scheduling while atomic: cc1/0x00000002/29745, CPU#0
> >  [<c03146c8>] __sched_text_start+0xb0/0x4fe
> >  [<c0116432>] try_to_wake_up+0x31b/0x326
> >  [<c013a3f4>] add_preempt_count+0x12/0xcc
> >  [<c013a3f4>] add_preempt_count+0x12/0xcc
> >  [<c0314bfc>] schedule+0xe6/0x100
> >  [<c03158db>] rt_spin_lock_slowlock+0xc5/0x145
> >  [<c0316027>] __rt_spin_lock+0x3a/0x3c
> >  [<c014102c>] get_zone_pcp+0x20/0x29
> >  [<c0141a40>] free_hot_cold_page+0xdc/0x167
> >  [<c013a3f4>] add_preempt_count+0x12/0xcc
> >  [<c0110d92>] pgd_dtor+0x0/0x1
> >  [<c015d865>] quicklist_trim+0xb7/0xe3
> >  [<c0111025>] check_pgt_cache+0x19/0x1c
> >  [<c0148df5>] free_pgtables+0x54/0x12c
> >  [<c013a3f4>] add_preempt_count+0x12/0xcc
> >  [<c014e5be>] unmap_region+0xeb/0x13b
> >  [<c014e861>] do_munmap+0xea/0xff
> >  [<c014e8a7>] sys_munmap+0x31/0x40
> >  [<c010276a>] syscall_call+0x7/0xb
> >  [<c0310000>] _shift_data_right_pages+0xb9/0xd1
> >  =======================
> > ---------------------------
> > | preempt count: 00000002 ]
> > | 2-level deep critical section nesting:
> > ----------------------------------------
> > .. [<c015d7c8>] .... quicklist_trim+0x1a/0xe3
> > .....[<00000000>] ..   ( <= _stext+0x3fefff50/0x14)
> > .. [<c031462b>] .... __sched_text_start+0x13/0x4fe
> > .....[<00000000>] ..   ( <= _stext+0x3fefff50/0x14)
> > 
> > > Signed-Off-By: Daniel Walker <dwalker@xxxxxxxxxx>
> > > 
> > > Index: linux-2.6.22.1/kernel/sched.c
> > > ===================================================================
> > > --- linux-2.6.22.1.orig/kernel/sched.c	2007-07-22 16:47:37.000000000 +0000
> > > +++ linux-2.6.22.1/kernel/sched.c	2007-07-22 16:16:48.000000000 +0000
> > > @@ -4900,7 +4900,6 @@ asmlinkage long sys_sched_yield(void)
> > >  	 * no need to preempt or enable interrupts:
> > >  	 */
> > >  	spin_unlock_no_resched(&rq->lock);
> > > -	rcu_read_unlock();
> > > 
> > >  	__schedule();
> > > 
> > > 
> > > 
> > > -
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> 
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