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. ]

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b1fffa2..6ce82c0 100644 (file)
--- 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);

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 7365ac5..3c19421 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -374,6 +374,8 @@ struct rcu_state {
                                                /*  but in jiffies. */
        unsigned long gp_activity;              /* Time of last GP kthread */
                                                /*  activity in jiffies. */
+       unsigned long gp_req_activity;          /* Time of last GP request */
+                                               /*  in jiffies. */
        unsigned long jiffies_stall;            /* Time at which to check */
                                                /*  for CPU stalls. */
        unsigned long jiffies_resched;          /* Time at which to resched */