rcu: Fix grace-period hangs due to race with CPU offline

Without special fail-safe quiescent-state-propagation checks, grace-period
hangs can result from the following scenario:

1.	CPU 1 goes offline.

2.	Because CPU 1 is the only CPU in the system blocking the current
	grace period, the grace period ends as soon as
	rcu_cleanup_dying_idle_cpu()'s call to rcu_report_qs_rnp()
	returns.

3.	At this point, the leaf rcu_node structure's ->lock is no longer
	held: rcu_report_qs_rnp() has released it, as it must in order
	to awaken the RCU grace-period kthread.

4.	At this point, that same leaf rcu_node structure's ->qsmaskinitnext
	field still records CPU 1 as being online.  This is absolutely
	necessary because the scheduler uses RCU (in this case on the
	wake-up path while awakening RCU's grace-period kthread), and
	->qsmaskinitnext contains RCU's idea as to which CPUs are online.
	Therefore, invoking rcu_report_qs_rnp() after clearing CPU 1's
	bit from ->qsmaskinitnext would result in a lockdep-RCU splat
	due to RCU being used from an offline CPU.

5.	RCU's grace-period kthread awakens, sees that the old grace period
	has completed and that a new one is needed.  It therefore starts
	a new grace period, but because CPU 1's leaf rcu_node structure's
	->qsmaskinitnext field still shows CPU 1 as being online, this new
	grace period is initialized to wait for a quiescent state from the
	now-offline CPU 1.

6.	Without the fail-safe force-quiescent-state checks, there would
	be no quiescent state from the now-offline CPU 1, which would
	eventually result in RCU CPU stall warnings and memory exhaustion.

It would be good to get rid of the special fail-safe quiescent-state
propagation checks, and thus it would be good to fix things so that
the above scenario cannot happen.  This commit therefore adds a new
->ofl_lock to the rcu_state structure.  This lock is held by rcu_gp_init()
across the applying of buffered online and offline operations to the
rcu_node tree, and it is also held by rcu_cleanup_dying_idle_cpu()
when buffering a new offline operation.  This prevents rcu_gp_init()
from acquiring the leaf rcu_node structure's lock during the interval
between when rcu_cleanup_dying_idle_cpu() invokes rcu_report_qs_rnp(),
which releases ->lock and the re-acquisition of that same lock.
This in turn prevents the failure scenario outlined above, and will
hopefully eventually allow removal of the offline-CPU checks from the
force-quiescent-state code path.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -101,6 +101,7 @@ struct rcu_state sname##_state = { \
 	.abbr = sabbr, \
 	.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
 	.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
+	.ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \
 }
 
 RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
@@ -1900,11 +1901,13 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 	 */
 	rcu_for_each_leaf_node(rsp, rnp) {
 		rcu_gp_slow(rsp, gp_preinit_delay);
+		spin_lock(&rsp->ofl_lock);
 		raw_spin_lock_irq_rcu_node(rnp);
 		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
 		    !rnp->wait_blkd_tasks) {
 			/* Nothing to do on this leaf rcu_node structure. */
 			raw_spin_unlock_irq_rcu_node(rnp);
+			spin_unlock(&rsp->ofl_lock);
 			continue;
 		}
 
@@ -1940,6 +1943,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 		}
 
 		raw_spin_unlock_irq_rcu_node(rnp);
+		spin_unlock(&rsp->ofl_lock);
 	}
 
 	/*
@@ -3749,6 +3753,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp)
 
 	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
 	mask = rdp->grpmask;
+	spin_lock(&rsp->ofl_lock);
 	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
 	if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
 		/* Report quiescent state -before- changing ->qsmaskinitnext! */
@@ -3757,6 +3762,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp)
 	}
 	rnp->qsmaskinitnext &= ~mask;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+	spin_unlock(&rsp->ofl_lock);
 }
 
 /*

kernel/rcu/tree.h

@@ -363,6 +363,10 @@ struct rcu_state {
 	const char *name;			/* Name of structure. */
 	char abbr;				/* Abbreviated name. */
 	struct list_head flavors;		/* List of RCU flavors. */
+
+	spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
+						/* Synchronize offline with */
+						/*  GP pre-initialization. */
 };
 
 /* Values for rcu_state structure's gp_flags field. */