rcutorture: Avoid problematic critical section nesting on RT
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> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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Scott Wood
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56ca6ad996
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a5850930e9
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@ -60,10 +60,13 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.ibm.com> and Josh Triplett <josh@
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#define RCUTORTURE_RDR_RBH 0x08 /* ... rcu_read_lock_bh(). */
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#define RCUTORTURE_RDR_SCHED 0x10 /* ... rcu_read_lock_sched(). */
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#define RCUTORTURE_RDR_RCU 0x20 /* ... entering another RCU reader. */
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#define RCUTORTURE_RDR_NBITS 6 /* Number of bits defined above. */
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#define RCUTORTURE_RDR_ATOM_BH 0x40 /* ... disabling bh while atomic */
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#define RCUTORTURE_RDR_ATOM_RBH 0x80 /* ... RBH while atomic */
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#define RCUTORTURE_RDR_NBITS 8 /* Number of bits defined above. */
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#define RCUTORTURE_MAX_EXTEND \
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(RCUTORTURE_RDR_BH | RCUTORTURE_RDR_IRQ | RCUTORTURE_RDR_PREEMPT | \
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RCUTORTURE_RDR_RBH | RCUTORTURE_RDR_SCHED)
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RCUTORTURE_RDR_RBH | RCUTORTURE_RDR_SCHED | \
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RCUTORTURE_RDR_ATOM_BH | RCUTORTURE_RDR_ATOM_RBH)
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#define RCUTORTURE_RDR_MAX_LOOPS 0x7 /* Maximum reader extensions. */
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/* Must be power of two minus one. */
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#define RCUTORTURE_RDR_MAX_SEGS (RCUTORTURE_RDR_MAX_LOOPS + 3)
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@ -1152,31 +1155,52 @@ static void rcutorture_one_extend(int *readstate, int newstate,
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WARN_ON_ONCE((idxold >> RCUTORTURE_RDR_SHIFT) > 1);
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rtrsp->rt_readstate = newstate;
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/* First, put new protection in place to avoid critical-section gap. */
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/*
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* First, put new protection in place to avoid critical-section gap.
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* Disable preemption around the ATOM disables to ensure that
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* in_atomic() is true.
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*/
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if (statesnew & RCUTORTURE_RDR_BH)
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local_bh_disable();
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if (statesnew & RCUTORTURE_RDR_RBH)
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rcu_read_lock_bh();
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if (statesnew & RCUTORTURE_RDR_IRQ)
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local_irq_disable();
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if (statesnew & RCUTORTURE_RDR_PREEMPT)
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preempt_disable();
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if (statesnew & RCUTORTURE_RDR_RBH)
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rcu_read_lock_bh();
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if (statesnew & RCUTORTURE_RDR_SCHED)
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rcu_read_lock_sched();
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preempt_disable();
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if (statesnew & RCUTORTURE_RDR_ATOM_BH)
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local_bh_disable();
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if (statesnew & RCUTORTURE_RDR_ATOM_RBH)
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rcu_read_lock_bh();
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preempt_enable();
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if (statesnew & RCUTORTURE_RDR_RCU)
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idxnew = cur_ops->readlock() << RCUTORTURE_RDR_SHIFT;
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/* Next, remove old protection, irq first due to bh conflict. */
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/*
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* Next, remove old protection, in decreasing order of strength
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* to avoid unlock paths that aren't safe in the stronger
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* context. Disable preemption around the ATOM enables in
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* case the context was only atomic due to IRQ disabling.
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*/
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preempt_disable();
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if (statesold & RCUTORTURE_RDR_IRQ)
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local_irq_enable();
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if (statesold & RCUTORTURE_RDR_BH)
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if (statesold & RCUTORTURE_RDR_ATOM_BH)
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local_bh_enable();
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if (statesold & RCUTORTURE_RDR_ATOM_RBH)
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rcu_read_unlock_bh();
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preempt_enable();
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if (statesold & RCUTORTURE_RDR_PREEMPT)
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preempt_enable();
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if (statesold & RCUTORTURE_RDR_RBH)
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rcu_read_unlock_bh();
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if (statesold & RCUTORTURE_RDR_SCHED)
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rcu_read_unlock_sched();
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if (statesold & RCUTORTURE_RDR_BH)
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local_bh_enable();
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if (statesold & RCUTORTURE_RDR_RBH)
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rcu_read_unlock_bh();
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if (statesold & RCUTORTURE_RDR_RCU)
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cur_ops->readunlock(idxold >> RCUTORTURE_RDR_SHIFT);
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@ -1212,6 +1236,12 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
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int mask = rcutorture_extend_mask_max();
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unsigned long randmask1 = torture_random(trsp) >> 8;
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unsigned long randmask2 = randmask1 >> 3;
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unsigned long preempts = RCUTORTURE_RDR_PREEMPT | RCUTORTURE_RDR_SCHED;
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unsigned long preempts_irq = preempts | RCUTORTURE_RDR_IRQ;
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unsigned long nonatomic_bhs = RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
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unsigned long atomic_bhs = RCUTORTURE_RDR_ATOM_BH |
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RCUTORTURE_RDR_ATOM_RBH;
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unsigned long tmp;
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WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
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/* Mostly only one bit (need preemption!), sometimes lots of bits. */
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@ -1219,11 +1249,49 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
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mask = mask & randmask2;
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else
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mask = mask & (1 << (randmask2 % RCUTORTURE_RDR_NBITS));
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/* Can't enable bh w/irq disabled. */
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if ((mask & RCUTORTURE_RDR_IRQ) &&
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((!(mask & RCUTORTURE_RDR_BH) && (oldmask & RCUTORTURE_RDR_BH)) ||
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(!(mask & RCUTORTURE_RDR_RBH) && (oldmask & RCUTORTURE_RDR_RBH))))
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mask |= RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
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/*
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* Can't enable bh w/irq disabled.
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*/
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tmp = atomic_bhs | nonatomic_bhs;
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if (mask & RCUTORTURE_RDR_IRQ)
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mask |= oldmask & tmp;
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/*
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* Ideally these sequences would be detected in debug builds
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* (regardless of RT), but until then don't stop testing
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* them on non-RT.
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*/
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if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
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/*
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* Can't release the outermost rcu lock in an irq disabled
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* section without preemption also being disabled, if irqs
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* had ever been enabled during this RCU critical section
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* (could leak a special flag and delay reporting the qs).
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*/
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if ((oldmask & RCUTORTURE_RDR_RCU) &&
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(mask & RCUTORTURE_RDR_IRQ) &&
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!(mask & preempts))
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mask |= RCUTORTURE_RDR_RCU;
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/* Can't modify atomic bh in non-atomic context */
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if ((oldmask & atomic_bhs) && (mask & atomic_bhs) &&
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!(mask & preempts_irq)) {
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mask |= oldmask & preempts_irq;
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if (mask & RCUTORTURE_RDR_IRQ)
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mask |= oldmask & tmp;
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}
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if ((mask & atomic_bhs) && !(mask & preempts_irq))
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mask |= RCUTORTURE_RDR_PREEMPT;
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/* Can't modify non-atomic bh in atomic context */
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tmp = nonatomic_bhs;
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if (oldmask & preempts_irq)
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mask &= ~tmp;
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if ((oldmask | mask) & preempts_irq)
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mask |= oldmask & tmp;
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}
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return mask ?: RCUTORTURE_RDR_RCU;
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}
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