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Subject[PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT
Date
rcutorture was generating some nesting scenarios that are not
reasonable. Constrain the state selection to avoid them.

Example #1:

1. preempt_disable()
2. local_bh_disable()
3. preempt_enable()
4. local_bh_enable()

On PREEMPT_RT, BH disabling takes a local lock only when called in
non-atomic context. Thus, atomic context must be retained until after BH
is re-enabled. Likewise, if BH is initially disabled in non-atomic
context, it cannot be re-enabled in atomic context.

Example #2:

1. rcu_read_lock()
2. local_irq_disable()
3. rcu_read_unlock()
4. local_irq_enable()

If the thread is preempted between steps 1 and 2,
rcu_read_unlock_special.b.blocked will be set, but it won't be
acted on in step 3 because IRQs are disabled. Thus, reporting of the
quiescent state will be delayed beyond the local_irq_enable().

For now, these scenarios will continue to be tested on non-PREEMPT_RT
kernels, until debug checks are added to ensure that they are not
happening elsewhere.

Signed-off-by: Scott Wood <swood@redhat.com>
---
v3: Limit to RT kernels, and remove one constraint that, while it
is bad on both RT and non-RT (missing a schedule), does not oops or
otherwise prevent using rcutorture. It wolud be added once debug checks
are implemented.

kernel/rcu/rcutorture.c | 96 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 82 insertions(+), 14 deletions(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index efaa5b3f4d3f..ecb82cc432af 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -60,10 +60,13 @@
#define RCUTORTURE_RDR_RBH 0x08 /* ... rcu_read_lock_bh(). */
#define RCUTORTURE_RDR_SCHED 0x10 /* ... rcu_read_lock_sched(). */
#define RCUTORTURE_RDR_RCU 0x20 /* ... entering another RCU reader. */
-#define RCUTORTURE_RDR_NBITS 6 /* Number of bits defined above. */
+#define RCUTORTURE_RDR_ATOM_BH 0x40 /* ... disabling bh while atomic */
+#define RCUTORTURE_RDR_ATOM_RBH 0x80 /* ... RBH while atomic */
+#define RCUTORTURE_RDR_NBITS 8 /* Number of bits defined above. */
#define RCUTORTURE_MAX_EXTEND \
(RCUTORTURE_RDR_BH | RCUTORTURE_RDR_IRQ | RCUTORTURE_RDR_PREEMPT | \
- RCUTORTURE_RDR_RBH | RCUTORTURE_RDR_SCHED)
+ RCUTORTURE_RDR_RBH | RCUTORTURE_RDR_SCHED | \
+ RCUTORTURE_RDR_ATOM_BH | RCUTORTURE_RDR_ATOM_RBH)
#define RCUTORTURE_RDR_MAX_LOOPS 0x7 /* Maximum reader extensions. */
/* Must be power of two minus one. */
#define RCUTORTURE_RDR_MAX_SEGS (RCUTORTURE_RDR_MAX_LOOPS + 3)
@@ -1092,31 +1095,52 @@ static void rcutorture_one_extend(int *readstate, int newstate,
WARN_ON_ONCE((idxold >> RCUTORTURE_RDR_SHIFT) > 1);
rtrsp->rt_readstate = newstate;

- /* First, put new protection in place to avoid critical-section gap. */
+ /*
+ * First, put new protection in place to avoid critical-section gap.
+ * Disable preemption around the ATOM disables to ensure that
+ * in_atomic() is true.
+ */
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();
+ preempt_disable();
+ if (statesnew & RCUTORTURE_RDR_ATOM_BH)
+ local_bh_disable();
+ if (statesnew & RCUTORTURE_RDR_ATOM_RBH)
+ rcu_read_lock_bh();
+ preempt_enable();
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. 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();
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)
cur_ops->readunlock(idxold >> RCUTORTURE_RDR_SHIFT);

@@ -1152,6 +1176,12 @@ static int rcutorture_extend_mask_max(void)
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 nonatomic_bhs = RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
+ unsigned long atomic_bhs = RCUTORTURE_RDR_ATOM_BH |
+ RCUTORTURE_RDR_ATOM_RBH;
+ unsigned long tmp;

WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
/* Mostly only one bit (need preemption!), sometimes lots of bits. */
@@ -1159,11 +1189,49 @@ static int rcutorture_extend_mask_max(void)
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.
+ */
+ tmp = atomic_bhs | nonatomic_bhs;
+ if (mask & RCUTORTURE_RDR_IRQ)
+ mask |= oldmask & tmp;
+
+ /*
+ * 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_FULL)) {
+ /*
+ * 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 atomic bh in non-atomic context */
+ if ((oldmask & atomic_bhs) && (mask & atomic_bhs) &&
+ !(mask & preempts_irq)) {
+ mask |= oldmask & preempts_irq;
+ if (mask & RCUTORTURE_RDR_IRQ)
+ mask |= oldmask & tmp;
+ }
+ if ((mask & atomic_bhs) && !(mask & preempts_irq))
+ mask |= RCUTORTURE_RDR_PREEMPT;
+
+ /* Can't modify non-atomic bh in atomic context */
+ tmp = nonatomic_bhs;
+ if (oldmask & preempts_irq)
+ mask &= ~tmp;
+ if ((oldmask | mask) & preempts_irq)
+ mask |= oldmask & tmp;
+ }
+
return mask ?: RCUTORTURE_RDR_RCU;
}

--
1.8.3.1
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