[PATCH] rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation

[Frederic Weisbecker <frederic@kernel.org>]

Hi,

I have found a potential race, but I might have missed something on the
way that makes it actually impossible... Please double check.

Thanks.

---
Subject: [PATCH] rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation

When rcu_barrier() calls rcu_rdp_cpu_online() and observes a CPU off
rnp->qsmaskinitnext, it means that all accesses from the offline CPU
preceding the CPUHP_TEARDOWN_CPU are visible to RCU barrier, including
callbacks expiration and counter updates.

However interrupts can still fire after stop_machine() re-enables
interrupts and before rcutree_report_cpu_dead(). The related accesses
happening between CPUHP_TEARDOWN_CPU and rnp->qsmaskinitnext clearing
are _NOT_ guaranteed to be seen by rcu_barrier() without proper
ordering, especially when callbacks are invoked there to the end, making
rcutree_migrate_callback() bypass barrier_lock.

The following theoretical race example can make rcu_barrier() hang:

CPU 0                                               CPU 1
-----                                               -----
//cpu_down()
smpboot_park_threads()
//ksoftirqd is parked now
<IRQ>
rcu_sched_clock_irq()
   invoke_rcu_core()
do_softirq()
   rcu_core()
      rcu_do_batch()
         // callback storm
         // rcu_do_batch() returns
         // before completing all
         // of them
   // do_softirq also returns early because of
   // timeout. It defers to ksoftirqd but
   // it's parked
</IRQ>
stop_machine()
   take_cpu_down()
                                                    rcu_barrier()
                                                        spin_lock(barrier_lock)
                                                        // observes rcu_segcblist_n_cbs(&rdp->cblist) != 0
<IRQ>
do_softirq()
   rcu_core()
      rcu_do_batch()
         //completes all pending callbacks
         //smp_mb() implied _after_ callback number dec
</IRQ>

rcutree_report_cpu_dead()
   rnp->qsmaskinitnext &= ~rdp->grpmask;

rcutree_migrate_callback()
   // no callback, early return without locking
   // barrier_lock
                                                        //observes !rcu_rdp_cpu_online(rdp)
                                                        rcu_barrier_entrain()
                                                           rcu_segcblist_entrain()
                                                              // Observe rcu_segcblist_n_cbs(rsclp) == 0
                                                              // because no barrier between reading
                                                              // rnp->qsmaskinitnext and rsclp->len
                                                              rcu_segcblist_add_len()
                                                                 smp_mb__before_atomic()
                                                                 // will now observe the 0 count and empty
                                                                 // list, but too late, we enqueue regardless
                                                                 WRITE_ONCE(rsclp->len, rsclp->len + v);
                                                        // ignored barrier callback
                                                        // rcu barrier stall...

This could be solved with a read memory barrier, enforcing the message
passing between rnp->qsmaskinitnext and rsclp->len, matching the full
memory barrier after rsclp->len addition in rcu_segcblist_add_len()
performed at the end of rcu_do_batch().

However the rcu_barrier() is complicated enough and probably doesn't
need too many more subtleties. CPU down is a slowpath and the
barrier_lock seldom contended. Solve the issue with unconditionally
locking the barrier_lock on rcutree_migrate_callbacks(). This makes sure
that either rcu_barrier() sees the empty queue or its entrained
callback will be migrated.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 944e55085262..925e006b64f9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4736,11 +4736,15 @@ void rcutree_migrate_callbacks(int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	bool needwake;
 
-	if (rcu_rdp_is_offloaded(rdp) ||
-	    rcu_segcblist_empty(&rdp->cblist))
-		return;  /* No callbacks to migrate. */
+	if (rcu_rdp_is_offloaded(rdp))
+		return;
 
 	raw_spin_lock_irqsave(&rcu_state.barrier_lock, flags);
+	if (rcu_segcblist_empty(&rdp->cblist)) {
+		raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags);
+		return;  /* No callbacks to migrate. */
+	}
+
 	WARN_ON_ONCE(rcu_rdp_cpu_online(rdp));
 	rcu_barrier_entrain(rdp);
 	my_rdp = this_cpu_ptr(&rcu_data);
-- 
2.34.1