Re: [PATCH] rcutorture: Avoid problematic critical section nesting on RT

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Aug 19, 2021 at 05:47:08PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-08-19 17:39:29 [+0200], To Paul E. McKenney wrote:
> > up with following which I can explain:
> > 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 40ef5417d9545..5c8b31b7eff03 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -1432,28 +1432,34 @@ static void rcutorture_one_extend(int *readstate, int newstate,
> >  	/* First, put new protection in place to avoid critical-section gap. */
> >  	if (statesnew & RCUTORTURE_RDR_BH)
> >  		local_bh_disable();
> > +	if (statesnew & RCUTORTURE_RDR_RBH)
> > +		rcu_read_lock_bh();
> >  	if (statesnew & RCUTORTURE_RDR_IRQ)
> >  		local_irq_disable();
> >  	if (statesnew & RCUTORTURE_RDR_PREEMPT)
> >  		preempt_disable();
> > -	if (statesnew & RCUTORTURE_RDR_RBH)
> > -		rcu_read_lock_bh();
> >  	if (statesnew & RCUTORTURE_RDR_SCHED)
> >  		rcu_read_lock_sched();
> >  	if (statesnew & RCUTORTURE_RDR_RCU)
> >  		idxnew = cur_ops->readlock() << RCUTORTURE_RDR_SHIFT;
> 
> So the ordering in the enable and disable part regarding BH is
> important. First BH, then preemption or IRQ.
> 
> > -	/* Next, remove old protection, irq first due to bh conflict. */
> > +	/*
> > +	 * Next, remove old protection, in decreasing order of strength
> > +	 * to avoid unlock paths that aren't safe in the stronger
> > +	 * context. Namely: BH can not be enabled with disabled interrupts.
> > +	 * Additionally PREEMPT_RT requires that BH is enabled in preemptible
> > +	 * context.
> > +	 */
> >  	if (statesold & RCUTORTURE_RDR_IRQ)
> >  		local_irq_enable();
> > -	if (statesold & RCUTORTURE_RDR_BH)
> > -		local_bh_enable();
> >  	if (statesold & RCUTORTURE_RDR_PREEMPT)
> >  		preempt_enable();
> > -	if (statesold & RCUTORTURE_RDR_RBH)
> > -		rcu_read_unlock_bh();
> >  	if (statesold & RCUTORTURE_RDR_SCHED)
> >  		rcu_read_unlock_sched();
> > +	if (statesold & RCUTORTURE_RDR_BH)
> > +		local_bh_enable();
> > +	if (statesold & RCUTORTURE_RDR_RBH)
> > +		rcu_read_unlock_bh();
> >  	if (statesold & RCUTORTURE_RDR_RCU) {
> >  		bool lockit = !statesnew && !(torture_random(trsp) & 0xffff);
> 
> The same in the unlock part so that BH is unlocked in preemptible
> context.
> Now if you need bh lock/unlock in atomic context (either with disabled
> IRQs or preemption) then I would dig out the atomic-bh part again and
> make !RT only without the preempt_disable() section around about which
> one you did complain.
> 
> > @@ -1496,6 +1502,9 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
> >  	int mask = rcutorture_extend_mask_max();
> >  	unsigned long randmask1 = torture_random(trsp) >> 8;
> >  	unsigned long randmask2 = randmask1 >> 3;
> > +	unsigned long preempts = RCUTORTURE_RDR_PREEMPT | RCUTORTURE_RDR_SCHED;
> > +	unsigned long preempts_irq = preempts | RCUTORTURE_RDR_IRQ;
> > +	unsigned long bhs = RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
> >  
> >  	WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
> >  	/* Mostly only one bit (need preemption!), sometimes lots of bits. */
> > @@ -1503,11 +1512,37 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
> >  		mask = mask & randmask2;
> >  	else
> >  		mask = mask & (1 << (randmask2 % RCUTORTURE_RDR_NBITS));
> > -	/* Can't enable bh w/irq disabled. */
> > -	if ((mask & RCUTORTURE_RDR_IRQ) &&
> > -	    ((!(mask & RCUTORTURE_RDR_BH) && (oldmask & RCUTORTURE_RDR_BH)) ||
> > -	     (!(mask & RCUTORTURE_RDR_RBH) && (oldmask & RCUTORTURE_RDR_RBH))))
> > -		mask |= RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
> > +
> > +	/*
> > +	 * Can't enable bh w/irq disabled.
> > +	 */
> > +	if (mask & RCUTORTURE_RDR_IRQ)
> > +		mask |= oldmask & bhs;
> > +
> > +	/*
> > +	 * Ideally these sequences would be detected in debug builds
> > +	 * (regardless of RT), but until then don't stop testing
> > +	 * them on non-RT.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		/*
> > +		 * Can't release the outermost rcu lock in an irq disabled
> > +		 * section without preemption also being disabled, if irqs
> > +		 * had ever been enabled during this RCU critical section
> > +		 * (could leak a special flag and delay reporting the qs).
> > +		 */
> > +		if ((oldmask & RCUTORTURE_RDR_RCU) &&
> > +		    (mask & RCUTORTURE_RDR_IRQ) &&
> > +		    !(mask & preempts))
> > +			mask |= RCUTORTURE_RDR_RCU;
> 
> This piece above, I don't understand. I had it running for a while and
> it didn't explode. Let me try TREE01 for 30min without that piece.

This might be historical.  There was a time when interrupts being
disabled across rcu_read_unlock() meant that preemption had to have
been disabled across the entire RCU read-side critical section.

I am not seeing a purpose for it now, but I could easily be missing
something, especially given my tenuous grasp of RT.

Either way, looking forward to the next version!

							Thanx, Paul

> > +		/* Can't modify bh in atomic context */
> > +		if (oldmask & preempts_irq)
> > +			mask &= ~bhs;
> > +		if ((oldmask | mask) & preempts_irq)
> > +			mask |= oldmask & bhs;
> 
> And this is needed because we can't lock/unlock bh while atomic.
> 
> > +	}
> > +
> >  	return mask ?: RCUTORTURE_RDR_RCU;
> >  }
> >  
> 
> Sebastian