rcutorture: Add self-propagating callback to forward-progress testing

If rcutorture is run on a quiet system with the rcutorture.stutter module
parameter set high, then there can legitimately be an extended period
during which no RCU forward progress takes place.  This can result
in false-positive no-forward-progress splats.  This commit therefore
makes rcu_torture_fwd_prog() create a self-propagating RCU callback
to ensure that grace periods are in progress for the duration of the
forward-progress test.

Note that the RCU flavor under test must define ->call(), ->sync(),
and ->cb_barrier() for this self-propagating callback to be created.
If one or more of those rcu_torture_ops fields are NULL, then the
rcu_torture_fwd_prog() function will silently proceed without creating
the self-propagating callback.

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

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 89cc4d9c9a0cdc1eaed1d2b6700b32d7876ebf5b..316083687fd7e6485c576169b0c770b6e49002b7 100644 (file)
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1670,20 +1670,49 @@ static int __init rcu_torture_stall_init(void)
        return torture_create_kthread(rcu_torture_stall, NULL, stall_task);
 }
 
+/* State structure for forward-progress self-propagating RCU callback. */
+struct fwd_cb_state {
+       struct rcu_head rh;
+       int stop;
+};
+
+/*
+ * Forward-progress self-propagating RCU callback function.  Because
+ * callbacks run from softirq, this function is an implicit RCU read-side
+ * critical section.
+ */
+static void rcu_torture_fwd_prog_cb(struct rcu_head *rhp)
+{
+       struct fwd_cb_state *fcsp = container_of(rhp, struct fwd_cb_state, rh);
+
+       if (READ_ONCE(fcsp->stop)) {
+               WRITE_ONCE(fcsp->stop, 2);
+               return;
+       }
+       cur_ops->call(&fcsp->rh, rcu_torture_fwd_prog_cb);
+}
+
 /* Carry out grace-period forward-progress testing. */
 static int rcu_torture_fwd_prog(void *args)
 {
        unsigned long cver;
+       struct fwd_cb_state fcs = { .stop = 0 };
        unsigned long gps;
        int idx;
        int sd;
        int sd4;
+       bool selfpropcb = false;
        unsigned long stopat;
        bool tested = false;
        int tested_tries = 0;
        static DEFINE_TORTURE_RANDOM(trs);
 
        VERBOSE_TOROUT_STRING("rcu_torture_fwd_progress task started");
+       if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+               init_rcu_head_on_stack(&fcs.rh);
+               cur_ops->call(&fcs.rh, rcu_torture_fwd_prog_cb);
+               selfpropcb = true;
+       }
        do {
                schedule_timeout_interruptible(fwd_progress_holdoff * HZ);
                cver = READ_ONCE(rcu_torture_current_version);
@@ -1708,6 +1737,13 @@ static int rcu_torture_fwd_prog(void *args)
                /* Avoid slow periods, better to test when busy. */
                stutter_wait("rcu_torture_fwd_prog");
        } while (!torture_must_stop());
+       if (selfpropcb) {
+               WRITE_ONCE(fcs.stop, 1);
+               cur_ops->sync(); /* Wait for running callback to complete. */
+               cur_ops->cb_barrier(); /* Wait for queued callbacks. */
+               WARN_ON(READ_ONCE(fcs.stop) != 2);
+               destroy_rcu_head_on_stack(&fcs.rh);
+       }
        /* Short runs might not contain a valid forward-progress attempt. */
        WARN_ON(!tested && tested_tries >= 5);
        torture_kthread_stopping("rcu_torture_fwd_prog");