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

[Paul E. McKenney]

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.

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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