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Subject[PATCH tip/core/rcu 12/47] rcu: Simplify offline processing
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

Move ->qsmaskinit and blkd_tasks[] manipulation to the CPU_DYING
notifier. This simplifies the code by eliminating a potential
deadlock and by reducing the responsibilities of force_quiescent_state().
Also rename functions to make their connection to the CPU-hotplug
stages explicit.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 160 ++++++++++++++++++++++-------------------------
kernel/rcutree.h | 4 +-
kernel/rcutree_plugin.h | 25 ++++----
3 files changed, 90 insertions(+), 99 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index acf2d67..575f91d 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -943,6 +943,10 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
* in preparation for detecting the next grace period. The caller must hold
* the root node's ->lock, which is released before return. Hard irqs must
* be disabled.
+ *
+ * Note that it is legal for a dying CPU (which is marked as offline) to
+ * invoke this function. This can happen when the dying CPU reports its
+ * quiescent state.
*/
static void
rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
@@ -1245,118 +1249,101 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)

/*
* Move a dying CPU's RCU callbacks to online CPU's callback list.
- * Synchronization is not required because this function executes
- * in stop_machine() context.
+ * Also record a quiescent state for this CPU for the current grace period.
+ * Synchronization and interrupt disabling are not required because
+ * this function executes in stop_machine() context. Therefore, cleanup
+ * operations that might block must be done later from the CPU_DEAD
+ * notifier.
+ *
+ * Note that the outgoing CPU's bit has already been cleared in the
+ * cpu_online_mask. This allows us to randomly pick a callback
+ * destination from the bits set in that mask.
*/
-static void rcu_send_cbs_to_online(struct rcu_state *rsp)
+static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
+ unsigned long flags;
int i;
- /* current DYING CPU is cleared in the cpu_online_mask */
+ unsigned long mask;
+ int need_report;
int receive_cpu = cpumask_any(cpu_online_mask);
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
+ struct rcu_node *rnp = rdp->mynode; /* For dying CPU. */
+
+ /* Move callbacks to some other CPU. */
+ if (rdp->nxtlist != NULL) {
+ *receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
+ receive_rdp->nxttail[RCU_NEXT_TAIL] =
+ rdp->nxttail[RCU_NEXT_TAIL];
+ receive_rdp->qlen_lazy += rdp->qlen_lazy;
+ receive_rdp->qlen += rdp->qlen;
+ receive_rdp->n_cbs_adopted += rdp->qlen;
+ rdp->n_cbs_orphaned += rdp->qlen;
+
+ rdp->nxtlist = NULL;
+ for (i = 0; i < RCU_NEXT_SIZE; i++)
+ rdp->nxttail[i] = &rdp->nxtlist;
+ rdp->qlen_lazy = 0;
+ rdp->qlen = 0;
+ }

- if (rdp->nxtlist == NULL)
- return; /* irqs disabled, so comparison is stable. */
-
- *receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
- receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
- receive_rdp->qlen_lazy += rdp->qlen_lazy;
- receive_rdp->qlen += rdp->qlen;
- receive_rdp->n_cbs_adopted += rdp->qlen;
- rdp->n_cbs_orphaned += rdp->qlen;
-
- rdp->nxtlist = NULL;
- for (i = 0; i < RCU_NEXT_SIZE; i++)
- rdp->nxttail[i] = &rdp->nxtlist;
- rdp->qlen_lazy = 0;
- rdp->qlen = 0;
-}
-
-/*
- * Remove the outgoing CPU from the bitmasks in the rcu_node hierarchy
- * and move all callbacks from the outgoing CPU to the current one.
- * There can only be one CPU hotplug operation at a time, so no other
- * CPU can be attempting to update rcu_cpu_kthread_task.
- */
-static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
-{
- unsigned long flags;
- unsigned long mask;
- int need_report = 0;
- struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
- struct rcu_node *rnp;
-
- rcu_stop_cpu_kthread(cpu);
-
- /* Exclude any attempts to start a new grace period. */
- raw_spin_lock_irqsave(&rsp->onofflock, flags);
-
- /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
- rnp = rdp->mynode; /* this is the outgoing CPU's rnp. */
+ /* Record a quiescent state for the dying CPU. */
mask = rdp->grpmask; /* rnp->grplo is constant. */
+ trace_rcu_grace_period(rsp->name,
+ rnp->gpnum + 1 - !!(rnp->qsmask & mask),
+ "cpuofl");
+ rcu_report_qs_rdp(smp_processor_id(), rsp, rdp, rsp->gpnum);
+ /* Note that rcu_report_qs_rdp() might call trace_rcu_grace_period(). */
+
+ /*
+ * Remove the dying CPU from the bitmasks in the rcu_node
+ * hierarchy. Because we are in stop_machine() context, we
+ * automatically exclude ->onofflock critical sections.
+ */
do {
- raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+ raw_spin_lock_irqsave(&rnp->lock, flags);
rnp->qsmaskinit &= ~mask;
if (rnp->qsmaskinit != 0) {
- if (rnp != rdp->mynode)
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
- else
- trace_rcu_grace_period(rsp->name,
- rnp->gpnum + 1 -
- !!(rnp->qsmask & mask),
- "cpuofl");
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
break;
}
if (rnp == rdp->mynode) {
- trace_rcu_grace_period(rsp->name,
- rnp->gpnum + 1 -
- !!(rnp->qsmask & mask),
- "cpuofl");
need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
+ if (need_report & RCU_OFL_TASKS_NORM_GP)
+ rcu_report_unblock_qs_rnp(rnp, flags);
+ else
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ if (need_report & RCU_OFL_TASKS_EXP_GP)
+ rcu_report_exp_rnp(rsp, rnp, true);
} else
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
mask = rnp->grpmask;
rnp = rnp->parent;
} while (rnp != NULL);
-
- /*
- * We still hold the leaf rcu_node structure lock here, and
- * irqs are still disabled. The reason for this subterfuge is
- * because invoking rcu_report_unblock_qs_rnp() with ->onofflock
- * held leads to deadlock.
- */
- raw_spin_unlock(&rsp->onofflock); /* irqs remain disabled. */
- rnp = rdp->mynode;
- if (need_report & RCU_OFL_TASKS_NORM_GP)
- rcu_report_unblock_qs_rnp(rnp, flags);
- else
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- if (need_report & RCU_OFL_TASKS_EXP_GP)
- rcu_report_exp_rnp(rsp, rnp, true);
- rcu_node_kthread_setaffinity(rnp, -1);
}

/*
- * Remove the specified CPU from the RCU hierarchy and move any pending
- * callbacks that it might have to the current CPU. This code assumes
- * that at least one CPU in the system will remain running at all times.
- * Any attempt to offline -all- CPUs is likely to strand RCU callbacks.
+ * The CPU has been completely removed, and some other CPU is reporting
+ * this fact from process context. Do the remainder of the cleanup.
+ * There can only be one CPU hotplug operation at a time, so no other
+ * CPU can be attempting to update rcu_cpu_kthread_task.
*/
-static void rcu_offline_cpu(int cpu)
+static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
{
- __rcu_offline_cpu(cpu, &rcu_sched_state);
- __rcu_offline_cpu(cpu, &rcu_bh_state);
- rcu_preempt_offline_cpu(cpu);
+ struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
+ struct rcu_node *rnp = rdp->mynode;
+
+ rcu_stop_cpu_kthread(cpu);
+ rcu_node_kthread_setaffinity(rnp, -1);
}

#else /* #ifdef CONFIG_HOTPLUG_CPU */

-static void rcu_send_cbs_to_online(struct rcu_state *rsp)
+static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
}

-static void rcu_offline_cpu(int cpu)
+static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
{
}

@@ -1725,6 +1712,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
* a quiescent state betweentimes.
*/
local_irq_save(flags);
+ WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
rdp = this_cpu_ptr(rsp->rda);

/* Add the callback to our list. */
@@ -2155,16 +2143,18 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
* touch any data without introducing corruption. We send the
* dying CPU's callbacks to an arbitrarily chosen online CPU.
*/
- rcu_send_cbs_to_online(&rcu_bh_state);
- rcu_send_cbs_to_online(&rcu_sched_state);
- rcu_preempt_send_cbs_to_online();
+ rcu_cleanup_dying_cpu(&rcu_bh_state);
+ rcu_cleanup_dying_cpu(&rcu_sched_state);
+ rcu_preempt_cleanup_dying_cpu();
rcu_cleanup_after_idle(cpu);
break;
case CPU_DEAD:
case CPU_DEAD_FROZEN:
case CPU_UP_CANCELED:
case CPU_UP_CANCELED_FROZEN:
- rcu_offline_cpu(cpu);
+ rcu_cleanup_dead_cpu(cpu, &rcu_bh_state);
+ rcu_cleanup_dead_cpu(cpu, &rcu_sched_state);
+ rcu_preempt_cleanup_dead_cpu(cpu);
break;
default:
break;
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index af2af3c..05e0367 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -439,8 +439,8 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp);
static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
struct rcu_node *rnp,
struct rcu_data *rdp);
-static void rcu_preempt_offline_cpu(int cpu);
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
+static void rcu_preempt_cleanup_dead_cpu(int cpu);
static void rcu_preempt_check_callbacks(int cpu);
static void rcu_preempt_process_callbacks(void);
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
@@ -451,7 +451,7 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
static int rcu_preempt_pending(int cpu);
static int rcu_preempt_needs_cpu(int cpu);
static void __cpuinit rcu_preempt_init_percpu_data(int cpu);
-static void rcu_preempt_send_cbs_to_online(void);
+static void rcu_preempt_cleanup_dying_cpu(void);
static void __init __rcu_init_preempt(void);
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 7adf232..eeb2cc6 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -618,16 +618,16 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
return retval;
}

+#endif /* #ifdef CONFIG_HOTPLUG_CPU */
+
/*
* Do CPU-offline processing for preemptible RCU.
*/
-static void rcu_preempt_offline_cpu(int cpu)
+static void rcu_preempt_cleanup_dead_cpu(int cpu)
{
- __rcu_offline_cpu(cpu, &rcu_preempt_state);
+ rcu_cleanup_dead_cpu(cpu, &rcu_preempt_state);
}

-#endif /* #ifdef CONFIG_HOTPLUG_CPU */
-
/*
* Check for a quiescent state from the current CPU. When a task blocks,
* the task is recorded in the corresponding CPU's rcu_node structure,
@@ -912,11 +912,12 @@ static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
}

/*
- * Move preemptible RCU's callbacks from dying CPU to other online CPU.
+ * Move preemptible RCU's callbacks from dying CPU to other online CPU
+ * and record a quiescent state.
*/
-static void rcu_preempt_send_cbs_to_online(void)
+static void rcu_preempt_cleanup_dying_cpu(void)
{
- rcu_send_cbs_to_online(&rcu_preempt_state);
+ rcu_cleanup_dying_cpu(&rcu_preempt_state);
}

/*
@@ -1052,16 +1053,16 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
return 0;
}

+#endif /* #ifdef CONFIG_HOTPLUG_CPU */
+
/*
* Because preemptible RCU does not exist, it never needs CPU-offline
* processing.
*/
-static void rcu_preempt_offline_cpu(int cpu)
+static void rcu_preempt_cleanup_dead_cpu(int cpu)
{
}

-#endif /* #ifdef CONFIG_HOTPLUG_CPU */
-
/*
* Because preemptible RCU does not exist, it never has any callbacks
* to check.
@@ -1153,9 +1154,9 @@ static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
}

/*
- * Because there is no preemptible RCU, there are no callbacks to move.
+ * Because there is no preemptible RCU, there is no cleanup to do.
*/
-static void rcu_preempt_send_cbs_to_online(void)
+static void rcu_preempt_cleanup_dying_cpu(void)
{
}

--
1.7.8


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