1 files changed, 709 insertions, 0 deletions

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
new file mode 100644
index 000000000000..f65a73a97323
--- /dev/null
+++ b/kernel/rcu/tree_stall.h
@@ -0,0 +1,709 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * RCU CPU stall warnings for normal RCU grace periods
+ *
+ * Copyright IBM Corporation, 2019
+ *
+ * Author: Paul E. McKenney <paulmck@linux.ibm.com>
+ */
+
+//////////////////////////////////////////////////////////////////////////////
+//
+// Controlling CPU stall warnings, including delay calculation.
+
+/* panic() on RCU Stall sysctl. */
+int sysctl_panic_on_rcu_stall __read_mostly;
+
+#ifdef CONFIG_PROVE_RCU
+#define RCU_STALL_DELAY_DELTA          (5 * HZ)
+#else
+#define RCU_STALL_DELAY_DELTA          0
+#endif
+
+/* Limit-check stall timeouts specified at boottime and runtime. */
+int rcu_jiffies_till_stall_check(void)
+{
+        int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
+
+        /*
+         * Limit check must be consistent with the Kconfig limits
+         * for CONFIG_RCU_CPU_STALL_TIMEOUT.
+         */
+        if (till_stall_check < 3) {
+                WRITE_ONCE(rcu_cpu_stall_timeout, 3);
+                till_stall_check = 3;
+        } else if (till_stall_check > 300) {
+                WRITE_ONCE(rcu_cpu_stall_timeout, 300);
+                till_stall_check = 300;
+        }
+        return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
+}
+EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
+
+/* Don't do RCU CPU stall warnings during long sysrq printouts. */
+void rcu_sysrq_start(void)
+{
+        if (!rcu_cpu_stall_suppress)
+                rcu_cpu_stall_suppress = 2;
+}
+
+void rcu_sysrq_end(void)
+{
+        if (rcu_cpu_stall_suppress == 2)
+                rcu_cpu_stall_suppress = 0;
+}
+
+/* Don't print RCU CPU stall warnings during a kernel panic. */
+static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
+{
+        rcu_cpu_stall_suppress = 1;
+        return NOTIFY_DONE;
+}
+
+static struct notifier_block rcu_panic_block = {
+        .notifier_call = rcu_panic,
+};
+
+static int __init check_cpu_stall_init(void)
+{
+        atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
+        return 0;
+}
+early_initcall(check_cpu_stall_init);
+
+/* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
+static void panic_on_rcu_stall(void)
+{
+        if (sysctl_panic_on_rcu_stall)
+                panic("RCU Stall\n");
+}
+
+/**
+ * rcu_cpu_stall_reset - prevent further stall warnings in current grace period
+ *
+ * Set the stall-warning timeout way off into the future, thus preventing
+ * any RCU CPU stall-warning messages from appearing in the current set of
+ * RCU grace periods.
+ *
+ * The caller must disable hard irqs.
+ */
+void rcu_cpu_stall_reset(void)
+{
+        WRITE_ONCE(rcu_state.jiffies_stall, jiffies + ULONG_MAX / 2);
+}
+
+//////////////////////////////////////////////////////////////////////////////
+//
+// Interaction with RCU grace periods
+
+/* Start of new grace period, so record stall time (and forcing times). */
+static void record_gp_stall_check_time(void)
+{
+        unsigned long j = jiffies;
+        unsigned long j1;
+
+        rcu_state.gp_start = j;
+        j1 = rcu_jiffies_till_stall_check();
+        /* Record ->gp_start before ->jiffies_stall. */
+        smp_store_release(&rcu_state.jiffies_stall, j + j1); /* ^^^ */
+        rcu_state.jiffies_resched = j + j1 / 2;
+        rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
+}
+
+/* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
+static void zero_cpu_stall_ticks(struct rcu_data *rdp)
+{
+        rdp->ticks_this_gp = 0;
+        rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
+        WRITE_ONCE(rdp->last_fqs_resched, jiffies);
+}
+
+/*
+ * If too much time has passed in the current grace period, and if
+ * so configured, go kick the relevant kthreads.
+ */
+static void rcu_stall_kick_kthreads(void)
+{
+        unsigned long j;
+
+        if (!rcu_kick_kthreads)
+                return;
+        j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
+        if (time_after(jiffies, j) && rcu_state.gp_kthread &&
+            (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
+                WARN_ONCE(1, "Kicking %s grace-period kthread\n",
+                          rcu_state.name);
+                rcu_ftrace_dump(DUMP_ALL);
+                wake_up_process(rcu_state.gp_kthread);
+                WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
+        }
+}
+
+/*
+ * Handler for the irq_work request posted about halfway into the RCU CPU
+ * stall timeout, and used to detect excessive irq disabling.  Set state
+ * appropriately, but just complain if there is unexpected state on entry.
+ */
+static void rcu_iw_handler(struct irq_work *iwp)
+{
+        struct rcu_data *rdp;
+        struct rcu_node *rnp;
+
+        rdp = container_of(iwp, struct rcu_data, rcu_iw);
+        rnp = rdp->mynode;
+        raw_spin_lock_rcu_node(rnp);
+        if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
+                rdp->rcu_iw_gp_seq = rnp->gp_seq;
+                rdp->rcu_iw_pending = false;
+        }
+        raw_spin_unlock_rcu_node(rnp);
+}
+
+//////////////////////////////////////////////////////////////////////////////
+//
+// Printing RCU CPU stall warnings
+
+#ifdef CONFIG_PREEMPT
+
+/*
+ * Dump detailed information for all tasks blocking the current RCU
+ * grace period on the specified rcu_node structure.
+ */
+static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
+{
+        unsigned long flags;
+        struct task_struct *t;
+
+        raw_spin_lock_irqsave_rcu_node(rnp, flags);
+        if (!rcu_preempt_blocked_readers_cgp(rnp)) {
+                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+                return;
+        }
+        t = list_entry(rnp->gp_tasks->prev,
+                       struct task_struct, rcu_node_entry);
+        list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
+                /*
+                 * We could be printing a lot while holding a spinlock.
+                 * Avoid triggering hard lockup.
+                 */
+                touch_nmi_watchdog();
+                sched_show_task(t);
+        }
+        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+}
+
+/*
+ * Scan the current list of tasks blocked within RCU read-side critical
+ * sections, printing out the tid of each.
+ */
+static int rcu_print_task_stall(struct rcu_node *rnp)
+{
+        struct task_struct *t;
+        int ndetected = 0;
+
+        if (!rcu_preempt_blocked_readers_cgp(rnp))
+                return 0;
+        pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
+               rnp->level, rnp->grplo, rnp->grphi);
+        t = list_entry(rnp->gp_tasks->prev,
+                       struct task_struct, rcu_node_entry);
+        list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
+                pr_cont(" P%d", t->pid);
+                ndetected++;
+        }
+        pr_cont("\n");
+        return ndetected;
+}
+
+#else /* #ifdef CONFIG_PREEMPT */
+
+/*
+ * Because preemptible RCU does not exist, we never have to check for
+ * tasks blocked within RCU read-side critical sections.
+ */
+static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
+{
+}
+
+/*
+ * Because preemptible RCU does not exist, we never have to check for
+ * tasks blocked within RCU read-side critical sections.
+ */
+static int rcu_print_task_stall(struct rcu_node *rnp)
+{
+        return 0;
+}
+#endif /* #else #ifdef CONFIG_PREEMPT */
+
+/*
+ * Dump stacks of all tasks running on stalled CPUs.  First try using
+ * NMIs, but fall back to manual remote stack tracing on architectures
+ * that don't support NMI-based stack dumps.  The NMI-triggered stack
+ * traces are more accurate because they are printed by the target CPU.
+ */
+static void rcu_dump_cpu_stacks(void)
+{
+        int cpu;
+        unsigned long flags;
+        struct rcu_node *rnp;
+
+        rcu_for_each_leaf_node(rnp) {
+                raw_spin_lock_irqsave_rcu_node(rnp, flags);
+                for_each_leaf_node_possible_cpu(rnp, cpu)
+                        if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+                                if (!trigger_single_cpu_backtrace(cpu))
+                                        dump_cpu_task(cpu);
+                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+        }
+}
+
+#ifdef CONFIG_RCU_FAST_NO_HZ
+
+static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
+{
+        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
+
+        sprintf(cp, "last_accelerate: %04lx/%04lx, Nonlazy posted: %c%c%c",
+                rdp->last_accelerate & 0xffff, jiffies & 0xffff,
+                ".l"[rdp->all_lazy],
+                ".L"[!rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)],
+                ".D"[!!rdp->tick_nohz_enabled_snap]);
+}
+
+#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
+
+static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
+{
+        *cp = '\0';
+}
+
+#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
+
+/*
+ * Print out diagnostic information for the specified stalled CPU.
+ *
+ * If the specified CPU is aware of the current RCU grace period, then
+ * print the number of scheduling clock interrupts the CPU has taken
+ * during the time that it has been aware.  Otherwise, print the number
+ * of RCU grace periods that this CPU is ignorant of, for example, "1"
+ * if the CPU was aware of the previous grace period.
+ *
+ * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
+ */
+static void print_cpu_stall_info(int cpu)
+{
+        unsigned long delta;
+        char fast_no_hz[72];
+        struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+        char *ticks_title;
+        unsigned long ticks_value;
+
+        /*
+         * We could be printing a lot while holding a spinlock.  Avoid
+         * triggering hard lockup.
+         */
+        touch_nmi_watchdog();
+
+        ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
+        if (ticks_value) {
+                ticks_title = "GPs behind";
+        } else {
+                ticks_title = "ticks this GP";
+                ticks_value = rdp->ticks_this_gp;
+        }
+        print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
+        delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
+        pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n",
+               cpu,
+               "O."[!!cpu_online(cpu)],
+               "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
+               "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
+               !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
+                        rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
+                                "!."[!delta],
+               ticks_value, ticks_title,
+               rcu_dynticks_snap(rdp) & 0xfff,
+               rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
+               rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
+               READ_ONCE(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
+               fast_no_hz);
+}
+
+/* Complain about starvation of grace-period kthread.  */
+static void rcu_check_gp_kthread_starvation(void)
+{
+        struct task_struct *gpk = rcu_state.gp_kthread;
+        unsigned long j;
+
+        j = jiffies - READ_ONCE(rcu_state.gp_activity);
+        if (j > 2 * HZ) {
+                pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx ->cpu=%d\n",
+                       rcu_state.name, j,
+                       (long)rcu_seq_current(&rcu_state.gp_seq),
+                       READ_ONCE(rcu_state.gp_flags),
+                       gp_state_getname(rcu_state.gp_state), rcu_state.gp_state,
+                       gpk ? gpk->state : ~0, gpk ? task_cpu(gpk) : -1);
+                if (gpk) {
+                        pr_err("RCU grace-period kthread stack dump:\n");
+                        sched_show_task(gpk);
+                        wake_up_process(gpk);
+                }
+        }
+}
+
+static void print_other_cpu_stall(unsigned long gp_seq)
+{
+        int cpu;
+        unsigned long flags;
+        unsigned long gpa;
+        unsigned long j;
+        int ndetected = 0;
+        struct rcu_node *rnp;
+        long totqlen = 0;
+
+        /* Kick and suppress, if so configured. */
+        rcu_stall_kick_kthreads();
+        if (rcu_cpu_stall_suppress)
+                return;
+
+        /*
+         * OK, time to rat on our buddy...
+         * See Documentation/RCU/stallwarn.txt for info on how to debug
+         * RCU CPU stall warnings.
+         */
+        pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
+        rcu_for_each_leaf_node(rnp) {
+                raw_spin_lock_irqsave_rcu_node(rnp, flags);
+                ndetected += rcu_print_task_stall(rnp);
+                if (rnp->qsmask != 0) {
+                        for_each_leaf_node_possible_cpu(rnp, cpu)
+                                if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
+                                        print_cpu_stall_info(cpu);
+                                        ndetected++;
+                                }
+                }
+                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+        }
+
+        for_each_possible_cpu(cpu)
+                totqlen += rcu_get_n_cbs_cpu(cpu);
+        pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
+               smp_processor_id(), (long)(jiffies - rcu_state.gp_start),
+               (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
+        if (ndetected) {
+                rcu_dump_cpu_stacks();
+
+                /* Complain about tasks blocking the grace period. */
+                rcu_for_each_leaf_node(rnp)
+                        rcu_print_detail_task_stall_rnp(rnp);
+        } else {
+                if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
+                        pr_err("INFO: Stall ended before state dump start\n");
+                } else {
+                        j = jiffies;
+                        gpa = READ_ONCE(rcu_state.gp_activity);
+                        pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
+                               rcu_state.name, j - gpa, j, gpa,
+                               READ_ONCE(jiffies_till_next_fqs),
+                               rcu_get_root()->qsmask);
+                        /* In this case, the current CPU might be at fault. */
+                        sched_show_task(current);
+                }
+        }
+        /* Rewrite if needed in case of slow consoles. */
+        if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
+                WRITE_ONCE(rcu_state.jiffies_stall,
+                           jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
+
+        rcu_check_gp_kthread_starvation();
+
+        panic_on_rcu_stall();
+
+        rcu_force_quiescent_state();  /* Kick them all. */
+}
+
+static void print_cpu_stall(void)
+{
+        int cpu;
+        unsigned long flags;
+        struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
+        struct rcu_node *rnp = rcu_get_root();
+        long totqlen = 0;
+
+        /* Kick and suppress, if so configured. */
+        rcu_stall_kick_kthreads();
+        if (rcu_cpu_stall_suppress)
+                return;
+
+        /*
+         * OK, time to rat on ourselves...
+         * See Documentation/RCU/stallwarn.txt for info on how to debug
+         * RCU CPU stall warnings.
+         */
+        pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
+        raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
+        print_cpu_stall_info(smp_processor_id());
+        raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
+        for_each_possible_cpu(cpu)
+                totqlen += rcu_get_n_cbs_cpu(cpu);
+        pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
+                jiffies - rcu_state.gp_start,
+                (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
+
+        rcu_check_gp_kthread_starvation();
+
+        rcu_dump_cpu_stacks();
+
+        raw_spin_lock_irqsave_rcu_node(rnp, flags);
+        /* Rewrite if needed in case of slow consoles. */
+        if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
+                WRITE_ONCE(rcu_state.jiffies_stall,
+                           jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
+        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+
+        panic_on_rcu_stall();
+
+        /*
+         * Attempt to revive the RCU machinery by forcing a context switch.
+         *
+         * A context switch would normally allow the RCU state machine to make
+         * progress and it could be we're stuck in kernel space without context
+         * switches for an entirely unreasonable amount of time.
+         */
+        set_tsk_need_resched(current);
+        set_preempt_need_resched();
+}
+
+static void check_cpu_stall(struct rcu_data *rdp)
+{
+        unsigned long gs1;
+        unsigned long gs2;
+        unsigned long gps;
+        unsigned long j;
+        unsigned long jn;
+        unsigned long js;
+        struct rcu_node *rnp;
+
+        if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
+            !rcu_gp_in_progress())
+                return;
+        rcu_stall_kick_kthreads();
+        j = jiffies;
+
+        /*
+         * Lots of memory barriers to reject false positives.
+         *
+         * The idea is to pick up rcu_state.gp_seq, then
+         * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
+         * another copy of rcu_state.gp_seq.  These values are updated in
+         * the opposite order with memory barriers (or equivalent) during
+         * grace-period initialization and cleanup.  Now, a false positive
+         * can occur if we get an new value of rcu_state.gp_start and a old
+         * value of rcu_state.jiffies_stall.  But given the memory barriers,
+         * the only way that this can happen is if one grace period ends
+         * and another starts between these two fetches.  This is detected
+         * by comparing the second fetch of rcu_state.gp_seq with the
+         * previous fetch from rcu_state.gp_seq.
+         *
+         * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
+         * and rcu_state.gp_start suffice to forestall false positives.
+         */
+        gs1 = READ_ONCE(rcu_state.gp_seq);
+        smp_rmb(); /* Pick up ->gp_seq first... */
+        js = READ_ONCE(rcu_state.jiffies_stall);
+        smp_rmb(); /* ...then ->jiffies_stall before the rest... */
+        gps = READ_ONCE(rcu_state.gp_start);
+        smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
+        gs2 = READ_ONCE(rcu_state.gp_seq);
+        if (gs1 != gs2 ||
+            ULONG_CMP_LT(j, js) ||
+            ULONG_CMP_GE(gps, js))
+                return; /* No stall or GP completed since entering function. */
+        rnp = rdp->mynode;
+        jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
+        if (rcu_gp_in_progress() &&
+            (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
+            cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
+
+                /* We haven't checked in, so go dump stack. */
+                print_cpu_stall();
+
+        } else if (rcu_gp_in_progress() &&
+                   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
+                   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
+
+                /* They had a few time units to dump stack, so complain. */
+                print_other_cpu_stall(gs2);
+        }
+}
+
+//////////////////////////////////////////////////////////////////////////////
+//
+// RCU forward-progress mechanisms, including of callback invocation.
+
+
+/*
+ * Show the state of the grace-period kthreads.
+ */
+void show_rcu_gp_kthreads(void)
+{
+        int cpu;
+        unsigned long j;
+        unsigned long ja;
+        unsigned long jr;
+        unsigned long jw;
+        struct rcu_data *rdp;
+        struct rcu_node *rnp;
+
+        j = jiffies;
+        ja = j - READ_ONCE(rcu_state.gp_activity);
+        jr = j - READ_ONCE(rcu_state.gp_req_activity);
+        jw = j - READ_ONCE(rcu_state.gp_wake_time);
+        pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
+                rcu_state.name, gp_state_getname(rcu_state.gp_state),
+                rcu_state.gp_state,
+                rcu_state.gp_kthread ? rcu_state.gp_kthread->state : 0x1ffffL,
+                ja, jr, jw, (long)READ_ONCE(rcu_state.gp_wake_seq),
+                (long)READ_ONCE(rcu_state.gp_seq),
+                (long)READ_ONCE(rcu_get_root()->gp_seq_needed),
+                READ_ONCE(rcu_state.gp_flags));
+        rcu_for_each_node_breadth_first(rnp) {
+                if (ULONG_CMP_GE(rcu_state.gp_seq, rnp->gp_seq_needed))
+                        continue;
+                pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
+                        rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
+                        (long)rnp->gp_seq_needed);
+                if (!rcu_is_leaf_node(rnp))
+                        continue;
+                for_each_leaf_node_possible_cpu(rnp, cpu) {
+                        rdp = per_cpu_ptr(&rcu_data, cpu);
+                        if (rdp->gpwrap ||
+                            ULONG_CMP_GE(rcu_state.gp_seq,
+                                         rdp->gp_seq_needed))
+                                continue;
+                        pr_info("\tcpu %d ->gp_seq_needed %ld\n",
+                                cpu, (long)rdp->gp_seq_needed);
+                }
+        }
+        /* sched_show_task(rcu_state.gp_kthread); */
+}
+EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
+
+/*
+ * 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_node *rnp, struct rcu_data *rdp,
+                                     const unsigned long gpssdelay)
+{
+        unsigned long flags;
+        unsigned long j;
+        struct rcu_node *rnp_root = rcu_get_root();
+        static atomic_t warned = ATOMIC_INIT(0);
+
+        if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
+            ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed))
+                return;
+        j = jiffies; /* Expensive access, and in common case don't get here. */
+        if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
+            time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
+            atomic_read(&warned))
+                return;
+
+        raw_spin_lock_irqsave_rcu_node(rnp, flags);
+        j = jiffies;
+        if (rcu_gp_in_progress() ||
+            ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
+            time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
+            time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
+            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() ||
+            ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
+            time_before(j, rcu_state.gp_req_activity + gpssdelay) ||
+            time_before(j, rcu_state.gp_activity + gpssdelay) ||
+            atomic_xchg(&warned, 1)) {
+                raw_spin_unlock_rcu_node(rnp_root); /* irqs remain disabled. */
+                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+                return;
+        }
+        WARN_ON(1);
+        if (rnp_root != rnp)
+                raw_spin_unlock_rcu_node(rnp_root);
+        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+        show_rcu_gp_kthreads();
+}
+
+/*
+ * Do a forward-progress check for rcutorture.  This is normally invoked
+ * due to an OOM event.  The argument "j" gives the time period during
+ * which rcutorture would like progress to have been made.
+ */
+void rcu_fwd_progress_check(unsigned long j)
+{
+        unsigned long cbs;
+        int cpu;
+        unsigned long max_cbs = 0;
+        int max_cpu = -1;
+        struct rcu_data *rdp;
+
+        if (rcu_gp_in_progress()) {
+                pr_info("%s: GP age %lu jiffies\n",
+                        __func__, jiffies - rcu_state.gp_start);
+                show_rcu_gp_kthreads();
+        } else {
+                pr_info("%s: Last GP end %lu jiffies ago\n",
+                        __func__, jiffies - rcu_state.gp_end);
+                preempt_disable();
+                rdp = this_cpu_ptr(&rcu_data);
+                rcu_check_gp_start_stall(rdp->mynode, rdp, j);
+                preempt_enable();
+        }
+        for_each_possible_cpu(cpu) {
+                cbs = rcu_get_n_cbs_cpu(cpu);
+                if (!cbs)
+                        continue;
+                if (max_cpu < 0)
+                        pr_info("%s: callbacks", __func__);
+                pr_cont(" %d: %lu", cpu, cbs);
+                if (cbs <= max_cbs)
+                        continue;
+                max_cbs = cbs;
+                max_cpu = cpu;
+        }
+        if (max_cpu >= 0)
+                pr_cont("\n");
+}
+EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
+
+/* Commandeer a sysrq key to dump RCU's tree. */
+static bool sysrq_rcu;
+module_param(sysrq_rcu, bool, 0444);
+
+/* Dump grace-period-request information due to commandeered sysrq. */
+static void sysrq_show_rcu(int key)
+{
+        show_rcu_gp_kthreads();
+}
+
+static struct sysrq_key_op sysrq_rcudump_op = {
+        .handler = sysrq_show_rcu,
+        .help_msg = "show-rcu(y)",
+        .action_msg = "Show RCU tree",
+        .enable_mask = SYSRQ_ENABLE_DUMP,
+};
+
+static int __init rcu_sysrq_init(void)
+{
+        if (sysrq_rcu)
+                return register_sysrq_key('y', &sysrq_rcudump_op);
+        return 0;
+}
+early_initcall(rcu_sysrq_init);