Subject: Re: linux-next ppc64: RCU mods cause __might_sleep BUGs

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 2 May 2012 14:36:59 -0700

On Wed, May 02, 2012 at 02:32:38PM -0700, Paul E. McKenney wrote:
> On Wed, May 02, 2012 at 01:49:54PM -0700, Paul E. McKenney wrote:
> > On Wed, May 02, 2012 at 01:25:30PM -0700, Hugh Dickins wrote:
> > > On Tue, 1 May 2012, Paul E. McKenney wrote:
> > > > > > > > On Mon, 2012-04-30 at 15:37 -0700, Hugh Dickins wrote:
> > > > > > > > > 
> > > > > > > > > BUG: sleeping function called from invalid context at include/linux/pagemap.h:354
> > > > > > > > > in_atomic(): 0, irqs_disabled(): 0, pid: 6886, name: cc1
> > > > > > > > > Call Trace:
> > > > > > > > > [c0000001a99f78e0] [c00000000000f34c] .show_stack+0x6c/0x16c (unreliable)
> > > > > > > > > [c0000001a99f7990] [c000000000077b40] .__might_sleep+0x11c/0x134
> > > > > > > > > [c0000001a99f7a10] [c0000000000c6228] .filemap_fault+0x1fc/0x494
> > > > > > > > > [c0000001a99f7af0] [c0000000000e7c9c] .__do_fault+0x120/0x684
> > > > > > > > > [c0000001a99f7c00] [c000000000025790] .do_page_fault+0x458/0x664
> > > > > > > > > [c0000001a99f7e30] [c000000000005868] handle_page_fault+0x10/0x30
> > > 
> > > Got it at last.  Embarrassingly obvious.  __rcu_read_lock() and
> > > __rcu_read_unlock() are not safe to be using __this_cpu operations,
> > > the cpu may change in between the rmw's read and write: they should
> > > be using this_cpu operations (or, I put preempt_disable/enable in the
> > > __rcu_read_unlock below).  __this_cpus there work out fine on x86,
> > > which was given good instructions to use; but not so well on PowerPC.
> > 
> > Thank you very much for tracking this down!!!
> > 
> > > I've been running successfully for an hour now with the patch below;
> > > but I expect you'll want to consider the tradeoffs, and may choose a
> > > different solution.
> > 
> > The thing that puzzles me about this is that the normal path through
> > the scheduler would save and restore these per-CPU variables to and
> > from the task structure.  There must be a sneak path through the
> > scheduler that I failed to account for.
> 
> Sigh...  I am slow today, I guess.  The preemption could of course
> happen between the time that the task calculated the address of the
> per-CPU variable and the time that it modified it.  If this happens,
> we are modifying some other CPU's per-CPU variable.
> 
> Given that Linus saw no performance benefit from this patchset, I am
> strongly tempted to just drop this inlinable-__rcu_read_lock() series
> at this point.
> 
> I suppose that the other option is to move preempt_count() to a
> per-CPU variable, then use the space in the task_info struct.
> But that didn't generate anywhere near as good of code...

But preempt_count() would suffer exactly the same problem.  The address
is calculated, the task moves to some other CPU, and then the task
is messing with some other CPU's preemption counter.  Blech.

							Thanx, Paul

> > But given your good work, this should be easy for me to chase down
> > even on my x86-based laptop -- just convert from __this_cpu_inc() to a
> > read-inc-delay-write sequence.  And check that the underlying variable
> > didn't change in the meantime.  And dump an ftrace if it did.  ;-)
> > 
> > Thank you again, Hugh!
> > 
> > 							Thanx, Paul
> > 
> > > Hugh
> > > 
> > > --- 3.4-rc4-next-20120427/include/linux/rcupdate.h	2012-04-28 09:26:38.000000000 -0700
> > > +++ testing/include/linux/rcupdate.h	2012-05-02 11:46:06.000000000 -0700
> > > @@ -159,7 +159,7 @@ DECLARE_PER_CPU(struct task_struct *, rc
> > >   */
> > >  static inline void __rcu_read_lock(void)
> > >  {
> > > -	__this_cpu_inc(rcu_read_lock_nesting);
> > > +	this_cpu_inc(rcu_read_lock_nesting);
> > >  	barrier(); /* Keep code within RCU read-side critical section. */
> > >  }
> > > 
> > > --- 3.4-rc4-next-20120427/kernel/rcupdate.c	2012-04-28 09:26:40.000000000 -0700
> > > +++ testing/kernel/rcupdate.c	2012-05-02 11:44:13.000000000 -0700
> > > @@ -72,6 +72,7 @@ DEFINE_PER_CPU(struct task_struct *, rcu
> > >   */
> > >  void __rcu_read_unlock(void)
> > >  {
> > > +	preempt_disable();
> > >  	if (__this_cpu_read(rcu_read_lock_nesting) != 1)
> > >  		__this_cpu_dec(rcu_read_lock_nesting);
> > >  	else {
> > > @@ -83,13 +84,14 @@ void __rcu_read_unlock(void)
> > >  		barrier();  /* ->rcu_read_unlock_special load before assign */
> > >  		__this_cpu_write(rcu_read_lock_nesting, 0);
> > >  	}
> > > -#ifdef CONFIG_PROVE_LOCKING
> > > +#if 1 /* CONFIG_PROVE_LOCKING */
> > >  	{
> > >  		int rln = __this_cpu_read(rcu_read_lock_nesting);
> > > 
> > > -		WARN_ON_ONCE(rln < 0 && rln > INT_MIN / 2);
> > > +		BUG_ON(rln < 0 && rln > INT_MIN / 2);
> > >  	}
> > >  #endif /* #ifdef CONFIG_PROVE_LOCKING */
> > > +	preempt_enable();
> > >  }
> > >  EXPORT_SYMBOL_GPL(__rcu_read_unlock);
> > > 
> > > --- 3.4-rc4-next-20120427/kernel/sched/core.c	2012-04-28 09:26:40.000000000 -0700
> > > +++ testing/kernel/sched/core.c	2012-05-01 22:40:46.000000000 -0700
> > > @@ -2024,7 +2024,7 @@ asmlinkage void schedule_tail(struct tas
> > >  {
> > >  	struct rq *rq = this_rq();
> > > 
> > > -	rcu_switch_from(prev);
> > > +	/* rcu_switch_from(prev); */
> > >  	rcu_switch_to();
> > >  	finish_task_switch(rq, prev);
> > > 
> > > @@ -7093,6 +7093,10 @@ void __might_sleep(const char *file, int
> > >  		"BUG: sleeping function called from invalid context at %s:%d\n",
> > >  			file, line);
> > >  	printk(KERN_ERR
> > > +		"cpu=%d preempt_count=%x preempt_offset=%x rcu_nesting=%x nesting_save=%x\n",
> > > +		raw_smp_processor_id(), preempt_count(), preempt_offset,
> > > +		rcu_preempt_depth(), current->rcu_read_lock_nesting_save); 
> > > +	printk(KERN_ERR
> > >  		"in_atomic(): %d, irqs_disabled(): %d, pid: %d, name: %s\n",
> > >  			in_atomic(), irqs_disabled(),
> > >  			current->pid, current->comm);
> > > --
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