rcutorture: Fully test lazy RCU

Currently, rcutorture bypasses lazy RCU by using call_rcu_hurry().
This works, avoiding the dreaded rtort_pipe_count WARN(), but fails to
fully test lazy RCU.  The rtort_pipe_count WARN() splats because lazy RCU
could delay the start of an RCU grace period for a full stutter period,
which defaults to only three seconds.

This commit therefore reverts the call_rcu_hurry() instances
back to call_rcu(), but, in kernels built with CONFIG_RCU_LAZY=y,
queues a workqueue handler just before the call to stutter_wait() in
rcu_torture_writer().  This workqueue handler invokes rcu_barrier(),
which motivates any lingering lazy callbacks, thus avoiding the splat.

Reported-by: Saravana Kannan <saravanak@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 5f2848b828dc423edddc80bb77465bd33a908714..882a158ada7bbd7bcd743e7997cc8bc4a6d5160e 100644 (file)
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -572,7 +572,7 @@ static unsigned long rcu_no_completed(void)
 
 static void rcu_torture_deferred_free(struct rcu_torture *p)
 {
-       call_rcu_hurry(&p->rtort_rcu, rcu_torture_cb);
+       call_rcu(&p->rtort_rcu, rcu_torture_cb);
 }
 
 static void rcu_sync_torture_init(void)
@@ -619,7 +619,7 @@ static struct rcu_torture_ops rcu_ops = {
        .poll_gp_state_exp      = poll_state_synchronize_rcu,
        .cond_sync_exp          = cond_synchronize_rcu_expedited,
        .cond_sync_exp_full     = cond_synchronize_rcu_expedited_full,
-       .call                   = call_rcu_hurry,
+       .call                   = call_rcu,
        .cb_barrier             = rcu_barrier,
        .fqs                    = rcu_force_quiescent_state,
        .gp_kthread_dbg         = show_rcu_gp_kthreads,
@@ -1145,7 +1145,7 @@ static void rcu_tasks_torture_deferred_free(struct rcu_torture *p)
 
 static void synchronize_rcu_mult_test(void)
 {
-       synchronize_rcu_mult(call_rcu_tasks, call_rcu_hurry);
+       synchronize_rcu_mult(call_rcu_tasks, call_rcu);
 }
 
 static struct rcu_torture_ops tasks_ops = {
@@ -1631,6 +1631,17 @@ static void do_rtws_sync(struct torture_random_state *trsp, void (*sync)(void))
                cpus_read_unlock();
 }
 
+/*
+ * Do an rcu_barrier() to motivate lazy callbacks during a stutter
+ * pause.  Without this, we can get false-positives rtort_pipe_count
+ * splats.
+ */
+static void rcu_torture_writer_work(struct work_struct *work)
+{
+       if (cur_ops->cb_barrier)
+               cur_ops->cb_barrier();
+}
+
 /*
  * RCU torture writer kthread.  Repeatedly substitutes a new structure
  * for that pointed to by rcu_torture_current, freeing the old structure
@@ -1651,6 +1662,7 @@ rcu_torture_writer(void *arg)
        int i;
        int idx;
        unsigned long j;
+       struct work_struct lazy_work;
        int oldnice = task_nice(current);
        struct rcu_gp_oldstate *rgo = NULL;
        int rgo_size = 0;
@@ -1703,6 +1715,9 @@ rcu_torture_writer(void *arg)
                pr_alert("%s" TORTURE_FLAG " Waited %lu jiffies for boot to complete.\n",
                         torture_type, jiffies - j);
 
+       if (IS_ENABLED(CONFIG_RCU_LAZY))
+               INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
+
        do {
                rcu_torture_writer_state = RTWS_FIXED_DELAY;
                torture_hrtimeout_us(500, 1000, &rand);
@@ -1895,6 +1910,8 @@ rcu_torture_writer(void *arg)
                                       !rcu_gp_is_normal();
                }
                rcu_torture_writer_state = RTWS_STUTTER;
+               if (IS_ENABLED(CONFIG_RCU_LAZY))
+                       queue_work(system_percpu_wq, &lazy_work);
                stutter_waited = stutter_wait("rcu_torture_writer");
                if (stutter_waited &&
                    !atomic_read(&rcu_fwd_cb_nodelay) &&
@@ -1925,6 +1942,12 @@ rcu_torture_writer(void *arg)
                pr_alert("%s" TORTURE_FLAG
                         " Dynamic grace-period expediting was disabled.\n",
                         torture_type);
+
+       if (IS_ENABLED(CONFIG_RCU_LAZY)) {
+               cancel_work_sync(&lazy_work);
+               destroy_work_on_stack(&lazy_work);
+       }
+
        kfree(ulo);
        kfree(rgo);
        rcu_torture_writer_state = RTWS_STOPPING;