rcu: Fix bug in rcu_barrier() torture test

The child threads in the rcu_torture_barrier_cbs() are improperly
synchronized, which can cause the rcu_barrier() tests to hang.  The
failure mode is as follows:

1.	CPU 0 running in rcu_torture_barrier() sets barrier_cbs_count
    	to n_barrier_cbs.

2.	CPU 1 running in rcu_torture_barrier_cbs() wakes up, posts
    	its RCU callback, and atomically decrements barrier_cbs_count.
    	Because barrier_cbs_count is not zero, it does not do the wake_up().

3.	CPU 2 running in rcu_torture_barrier_cbs() wakes up, but
    	finds that barrier_cbs_count is not equal to n_barrier_cbs,
    	and so returns to sleep.

4.	The value of barrier_cbs_count therefore never reaches zero,
    	which causes the test to hang.

This commit therefore uses a phase variable to coordinate the test,
preventing this scenario from occurring.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 7b6935e0cee3..f7fe73e59c9f 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -206,6 +206,7 @@ static unsigned long boost_starttime;	/* jiffies of next boost test start. */
 DEFINE_MUTEX(boost_mutex);              /* protect setting boost_starttime */
                                         /*  and boost task create/destroy. */
 static atomic_t barrier_cbs_count;      /* Barrier callbacks registered. */
+static bool barrier_phase;              /* Test phase. */
 static atomic_t barrier_cbs_invoked;    /* Barrier callbacks invoked. */
 static wait_queue_head_t *barrier_cbs_wq; /* Coordinate barrier testing. */
 static DECLARE_WAIT_QUEUE_HEAD(barrier_wq);
@@ -1642,6 +1643,7 @@ void rcu_torture_barrier_cbf(struct rcu_head *rcu)
 static int rcu_torture_barrier_cbs(void *arg)
 {
         long myid = (long)arg;
+        bool lastphase = 0;
         struct rcu_head rcu;
 
         init_rcu_head_on_stack(&rcu);
@@ -1649,9 +1651,11 @@ static int rcu_torture_barrier_cbs(void *arg)
         set_user_nice(current, 19);
         do {
                 wait_event(barrier_cbs_wq[myid],
-                           atomic_read(&barrier_cbs_count) == n_barrier_cbs ||
+                           barrier_phase != lastphase ||
                            kthread_should_stop() ||
                            fullstop != FULLSTOP_DONTSTOP);
+                lastphase = barrier_phase;
+                smp_mb(); /* ensure barrier_phase load before ->call(). */
                 if (kthread_should_stop() || fullstop != FULLSTOP_DONTSTOP)
                         break;
                 cur_ops->call(&rcu, rcu_torture_barrier_cbf);
@@ -1676,7 +1680,8 @@ static int rcu_torture_barrier(void *arg)
         do {
                 atomic_set(&barrier_cbs_invoked, 0);
                 atomic_set(&barrier_cbs_count, n_barrier_cbs);
-                /* wake_up() path contains the required barriers. */
+                smp_mb(); /* Ensure barrier_phase after prior assignments. */
+                barrier_phase = !barrier_phase;
                 for (i = 0; i < n_barrier_cbs; i++)
                         wake_up(&barrier_cbs_wq[i]);
                 wait_event(barrier_wq,