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

Sebastian Andrzej Siewior
2021-08-19T15:39:32Z

On 2021-08-18 15:46:51 [-0700], Paul E. McKenney wrote:
> > ---
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -61,10 +61,13 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck
…
> > - /* 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. Disable preemption around the ATOM enables in
> > + * case the context was only atomic due to IRQ disabling.
> > + */
> > + preempt_disable();
> > if (statesold & RCUTORTURE_RDR_IRQ)
> > local_irq_enable();
> > - if (statesold & RCUTORTURE_RDR_BH)
> > + if (statesold & RCUTORTURE_RDR_ATOM_BH)
> > local_bh_enable();
> > + if (statesold & RCUTORTURE_RDR_ATOM_RBH)
> > + rcu_read_unlock_bh();
> > + preempt_enable();
>
> The addition of preempt_enable() here prevents rcutorture from covering
> an important part of the mainline RCU state space, namely when an RCU
> read-side section ends with just local_irq_enable(). This situation
> is a challenge for RCU because it must indirectly detect the end of the
> critical section.
>
> Would it work for RT if the preempt_enable() and preempt_disable()
> were executed only if either RT on the one hand or statesold has the
> RCUTORTURE_RDR_ATOM_BH or RCUTORTURE_RDR_ATOM_RBH bit set on the other?

Now that I stared at it some more (and it stared briefly back at me) I
couldn't explain why we need this and that piece of the patch so I came
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;

- /* 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);

@@ -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;
+
+ /* Can't modify bh in atomic context */
+ if (oldmask & preempts_irq)
+ mask &= ~bhs;
+ if ((oldmask | mask) & preempts_irq)
+ mask |= oldmask & bhs;
+ }
+
return mask ?: RCUTORTURE_RDR_RCU;
}