This commit completes the process of consolidating the code for RCU CPU
stall warnings for normal grace periods by moving the remaining such
code from kernel/rcu/tree.c to kernel/rcu/tree_stall.h.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
kernel/rcu/tree.c | 291 ---------------------------------------
kernel/rcu/tree.h | 10 +-
kernel/rcu/tree_stall.h | 292 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 299 insertions(+), 294 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 424d50ccf9e6..001dd05f6e38 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -102,8 +102,6 @@ int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
/* Number of rcu_nodes at specified level. */
int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
-/* panic() on RCU Stall sysctl. */
-int sysctl_panic_on_rcu_stall __read_mostly;
/* Commandeer a sysrq key to dump RCU's tree. */
static bool sysrq_rcu;
module_param(sysrq_rcu, bool, 0444);
@@ -1167,295 +1165,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
return 0;
}
-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);
-}
-
-/*
- * 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);
- }
- }
-}
-
-/*
- * 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);
- }
-}
-
-/*
- * 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);
- }
-}
-
-static void panic_on_rcu_stall(void)
-{
- if (sysctl_panic_on_rcu_stall)
- panic("RCU Stall\n");
-}
-
-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 = 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 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:", rcu_state.name);
- print_cpu_stall_info_begin();
- 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);
- }
-
- print_cpu_stall_info_end();
- for_each_possible_cpu(cpu)
- totqlen += rcu_get_n_cbs_cpu(cpu);
- pr_cont("(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_print_detail_task_stall();
- } 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", rcu_state.name);
- print_cpu_stall_info_begin();
- 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);
- print_cpu_stall_info_end();
- for_each_possible_cpu(cpu)
- totqlen += rcu_get_n_cbs_cpu(cpu);
- pr_cont(" (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_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);
-}
-
/* Trace-event wrapper function for trace_rcu_future_grace_period. */
static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
unsigned long gp_seq_req, const char *s)
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index bb4f995f2d3f..3c4e26fff806 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -393,15 +393,13 @@ static const char *tp_rcu_varname __used __tracepoint_string = rcu_name;
int rcu_dynticks_snap(struct rcu_data *rdp);
-/* Forward declarations for rcutree_plugin.h */
+/* Forward declarations for tree_plugin.h */
static void rcu_bootup_announce(void);
static void rcu_qs(void);
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
#ifdef CONFIG_HOTPLUG_CPU
static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
-static void rcu_print_detail_task_stall(void);
-static int rcu_print_task_stall(struct rcu_node *rnp);
static int rcu_print_task_exp_stall(struct rcu_node *rnp);
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp);
static void rcu_flavor_sched_clock_irq(int user);
@@ -445,3 +443,9 @@ static void rcu_bind_gp_kthread(void);
static bool rcu_nohz_full_cpu(void);
static void rcu_dynticks_task_enter(void);
static void rcu_dynticks_task_exit(void);
+
+/* Forward declarations for tree_stall.h */
+static void rcu_print_detail_task_stall(void);
+static int rcu_print_task_stall(struct rcu_node *rnp);
+static void record_gp_stall_check_time(void);
+static void check_cpu_stall(struct rcu_data *rdp);
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 6f5f94944f49..e0e73f493363 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -8,6 +8,9 @@
*/
+/* 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
@@ -156,3 +159,292 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
return 0;
}
#endif /* #else #ifdef CONFIG_PREEMPT */
+
+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);
+}
+
+/*
+ * 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);
+ }
+ }
+}
+
+/*
+ * 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);
+ }
+}
+
+/*
+ * 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);
+ }
+}
+
+static void panic_on_rcu_stall(void)
+{
+ if (sysctl_panic_on_rcu_stall)
+ panic("RCU Stall\n");
+}
+
+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 = 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 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:", rcu_state.name);
+ print_cpu_stall_info_begin();
+ 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);
+ }
+
+ print_cpu_stall_info_end();
+ for_each_possible_cpu(cpu)
+ totqlen += rcu_get_n_cbs_cpu(cpu);
+ pr_cont("(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_print_detail_task_stall();
+ } 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", rcu_state.name);
+ print_cpu_stall_info_begin();
+ 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);
+ print_cpu_stall_info_end();
+ for_each_possible_cpu(cpu)
+ totqlen += rcu_get_n_cbs_cpu(cpu);
+ pr_cont(" (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_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);
+}
--
2.17.1
