rcu: Diagnostics for grace-period startup hangs

This commit causes a splat if RCU is idle and a request for a new grace
period is ignored for more than one second.  This splat normally indicates
that some code path asked for a new grace period, but failed to wake up
the RCU grace-period kthread.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix bug located by Dan Carpenter and his static checker. ]
[ paulmck: Fix self-deadlock bug located 0day test robot. ]
[ paulmck: Disable unless CONFIG_PROVE_RCU=y. ]

1 files changed, 64 insertions, 2 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b1fffa21b9e4..6ce82c009195 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1681,6 +1681,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
         }
         trace_rcu_this_gp(rnp_root, rdp, c, TPS("Startedroot"));
         WRITE_ONCE(rsp->gp_flags, rsp->gp_flags | RCU_GP_FLAG_INIT);
+        rsp->gp_req_activity = jiffies;
         if (!rsp->gp_kthread) {
                 trace_rcu_this_gp(rnp_root, rdp, c, TPS("NoGPkthread"));
                 goto unlock_out;
@@ -2113,6 +2114,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
         /* Advance CBs to reduce false positives below. */
         if (!rcu_accelerate_cbs(rsp, rnp, rdp) && needgp) {
                 WRITE_ONCE(rsp->gp_flags, RCU_GP_FLAG_INIT);
+                rsp->gp_req_activity = jiffies;
                 trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum),
                                        TPS("newreq"));
         }
@@ -2745,6 +2747,65 @@ static void force_quiescent_state(struct rcu_state *rsp)
 }
 
 /*
+ * This function checks for grace-period requests that fail to motivate
+ * RCU to come out of its idle mode.
+ */
+static void
+rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
+                         struct rcu_data *rdp)
+{
+        unsigned long flags;
+        unsigned long j;
+        struct rcu_node *rnp_root = rcu_get_root(rsp);
+        static atomic_t warned = ATOMIC_INIT(0);
+
+        if (!IS_ENABLED(CONFIG_PROVE_RCU) ||
+            rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)))
+                return;
+        j = jiffies; /* Expensive access, and in common case don't get here. */
+        if (time_before(j, READ_ONCE(rsp->gp_req_activity) + HZ) ||
+            time_before(j, READ_ONCE(rsp->gp_activity) + HZ) ||
+            atomic_read(&warned))
+                return;
+
+        raw_spin_lock_irqsave_rcu_node(rnp, flags);
+        j = jiffies;
+        if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)) ||
+            time_before(j, READ_ONCE(rsp->gp_req_activity) + HZ) ||
+            time_before(j, READ_ONCE(rsp->gp_activity) + HZ) ||
+            atomic_read(&warned)) {
+                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+                return;
+        }
+        /* Hold onto the leaf lock to make others see warned==1. */
+
+        if (rnp_root != rnp)
+                raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
+        j = jiffies;
+        if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)) ||
+            time_before(j, rsp->gp_req_activity + HZ) ||
+            time_before(j, rsp->gp_activity + HZ) ||
+            atomic_xchg(&warned, 1)) {
+                raw_spin_unlock_rcu_node(rnp_root); /* irqs remain disabled. */
+                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+                return;
+        }
+        pr_alert("%s: g%lu %d%d%d%d gar:%lu ga:%lu f%#x %s->state:%#lx\n",
+                 __func__, READ_ONCE(rsp->gpnum),
+                 need_future_gp_element(rcu_get_root(rsp), 0),
+                 need_future_gp_element(rcu_get_root(rsp), 1),
+                 need_future_gp_element(rcu_get_root(rsp), 2),
+                 need_future_gp_element(rcu_get_root(rsp), 3),
+                 j - rsp->gp_req_activity, j - rsp->gp_activity,
+                 rsp->gp_flags, rsp->name,
+                 rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL);
+        WARN_ON(1);
+        if (rnp_root != rnp)
+                raw_spin_unlock_rcu_node(rnp_root);
+        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+}
+
+/*
  * This does the RCU core processing work for the specified rcu_state
  * and rcu_data structures.  This may be called only from the CPU to
  * whom the rdp belongs.
@@ -2755,7 +2816,7 @@ __rcu_process_callbacks(struct rcu_state *rsp)
         unsigned long flags;
         bool needwake;
         struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
-        struct rcu_node *rnp;
+        struct rcu_node *rnp = rdp->mynode;
 
         WARN_ON_ONCE(!rdp->beenonline);
 
@@ -2769,7 +2830,6 @@ __rcu_process_callbacks(struct rcu_state *rsp)
                 if (rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL)) {
                         local_irq_restore(flags);
                 } else {
-                        rnp = rdp->mynode;
                         raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
                         needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
                         raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
@@ -2778,6 +2838,8 @@ __rcu_process_callbacks(struct rcu_state *rsp)
                 }
         }
 
+        rcu_check_gp_start_stall(rsp, rnp, rdp);
+
         /* If there are callbacks ready, invoke them. */
         if (rcu_segcblist_ready_cbs(&rdp->cblist))
                 invoke_rcu_callbacks(rsp, rdp);