rcu: Make rcu_barrier() less disruptive

The rcu_barrier() primitive interrupts each and every CPU, registering
a callback on every CPU.  Once all of these callbacks have been invoked,
rcu_barrier() knows that every callback that was registered before
the call to rcu_barrier() has also been invoked.

However, there is no point in registering a callback on a CPU that
currently has no callbacks, most especially if that CPU is in a
deep idle state.  This commit therefore makes rcu_barrier() avoid
interrupting CPUs that have no callbacks.  Doing this requires reworking
the handling of orphaned callbacks, otherwise callbacks could slip through
rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had
not yet interrupted to a CPU that rcu_barrier() had already interrupted.
This reworking was needed anyway to take a first step towards weaning
RCU from the CPU_DYING notifier's use of stop_cpu().

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

3 files changed, 222 insertions, 88 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 403306b86e7..e578dd327c6 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -75,6 +75,8 @@ static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];
         .gpnum = -300, \
         .completed = -300, \
         .onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \
+        .orphan_nxttail = &structname##_state.orphan_nxtlist, \
+        .orphan_donetail = &structname##_state.orphan_donelist, \
         .fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \
         .n_force_qs = 0, \
         .n_force_qs_ngp = 0, \
@@ -145,6 +147,13 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
 unsigned long rcutorture_testseq;
 unsigned long rcutorture_vernum;
 
+/* State information for rcu_barrier() and friends. */
+
+static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
+static atomic_t rcu_barrier_cpu_count;
+static DEFINE_MUTEX(rcu_barrier_mutex);
+static struct completion rcu_barrier_completion;
+
 /*
  * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
  * permit this function to be invoked without holding the root rcu_node
@@ -1311,95 +1320,133 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
 #ifdef CONFIG_HOTPLUG_CPU
 
 /*
- * Move a dying CPU's RCU callbacks to online CPU's callback list.
- * 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.
+ * Send the specified CPU's RCU callbacks to the orphanage.  The
+ * specified CPU must be offline, and the caller must hold the
+ * ->onofflock.
  */
-static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
+static void
+rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
+                          struct rcu_node *rnp, struct rcu_data *rdp)
 {
         int i;
-        unsigned long mask;
-        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);
-        RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */
 
-        /* First, adjust the counts. */
+        /*
+         * Orphan the callbacks.  First adjust the counts.  This is safe
+         * because ->onofflock excludes _rcu_barrier()'s adoption of
+         * the callbacks, thus no memory barrier is required.
+         */
         if (rdp->nxtlist != NULL) {
-                receive_rdp->qlen_lazy += rdp->qlen_lazy;
-                receive_rdp->qlen += rdp->qlen;
+                rsp->qlen_lazy += rdp->qlen_lazy;
+                rsp->qlen += rdp->qlen;
+                rdp->n_cbs_orphaned += rdp->qlen;
                 rdp->qlen_lazy = 0;
                 rdp->qlen = 0;
         }
 
         /*
-         * Next, move ready-to-invoke callbacks to be invoked on some
-         * other CPU.  These will not be required to pass through another
-         * grace period:  They are done, regardless of CPU.
+         * Next, move those callbacks still needing a grace period to
+         * the orphanage, where some other CPU will pick them up.
+         * Some of the callbacks might have gone partway through a grace
+         * period, but that is too bad.  They get to start over because we
+         * cannot assume that grace periods are synchronized across CPUs.
+         * We don't bother updating the ->nxttail[] array yet, instead
+         * we just reset the whole thing later on.
          */
-        if (rdp->nxtlist != NULL &&
-            rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
-                struct rcu_head *oldhead;
-                struct rcu_head **oldtail;
-                struct rcu_head **newtail;
-
-                oldhead = rdp->nxtlist;
-                oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
-                rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
-                *rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
-                *receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
-                newtail = rdp->nxttail[RCU_DONE_TAIL];
-                for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
-                        if (receive_rdp->nxttail[i] == oldtail)
-                                receive_rdp->nxttail[i] = newtail;
-                        if (rdp->nxttail[i] == newtail)
-                                rdp->nxttail[i] = &rdp->nxtlist;
-                }
+        if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
+                *rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
+                rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
+                *rdp->nxttail[RCU_DONE_TAIL] = NULL;
         }
 
         /*
-         * Finally, put the rest of the callbacks at the end of the list.
-         * The ones that made it partway through get to start over:  We
-         * cannot assume that grace periods are synchronized across CPUs.
-         * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
-         * this does not seem compelling.  Not yet, anyway.)
+         * Then move the ready-to-invoke callbacks to the orphanage,
+         * where some other CPU will pick them up.  These will not be
+         * required to pass though another grace period: They are done.
          */
         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->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;
+                *rsp->orphan_donetail = rdp->nxtlist;
+                rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
         }
 
+        /* Finally, initialize the rcu_data structure's list to empty.  */
+        rdp->nxtlist = NULL;
+        for (i = 0; i < RCU_NEXT_SIZE; i++)
+                rdp->nxttail[i] = &rdp->nxtlist;
+}
+
+/*
+ * Adopt the RCU callbacks from the specified rcu_state structure's
+ * orphanage.  The caller must hold the ->onofflock.
+ */
+static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
+{
+        int i;
+        struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
+
         /*
-         * Record a quiescent state for the dying CPU.  This is safe
-         * only because we have already cleared out the callbacks.
-         * (Otherwise, the RCU core might try to schedule the invocation
-         * of callbacks on this now-offline CPU, which would be bad.)
+         * If there is an rcu_barrier() operation in progress, then
+         * only the task doing that operation is permitted to adopt
+         * callbacks.  To do otherwise breaks rcu_barrier() and friends
+         * by causing them to fail to wait for the callbacks in the
+         * orphanage.
          */
-        mask = rdp->grpmask;    /* rnp->grplo is constant. */
+        if (rsp->rcu_barrier_in_progress &&
+            rsp->rcu_barrier_in_progress != current)
+                return;
+
+        /* Do the accounting first. */
+        rdp->qlen_lazy += rsp->qlen_lazy;
+        rdp->qlen += rsp->qlen;
+        rdp->n_cbs_adopted += rsp->qlen;
+        rsp->qlen_lazy = 0;
+        rsp->qlen = 0;
+
+        /*
+         * We do not need a memory barrier here because the only way we
+         * can get here if there is an rcu_barrier() in flight is if
+         * we are the task doing the rcu_barrier().
+         */
+
+        /* First adopt the ready-to-invoke callbacks. */
+        if (rsp->orphan_donelist != NULL) {
+                *rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
+                *rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
+                for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
+                        if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
+                                rdp->nxttail[i] = rsp->orphan_donetail;
+                rsp->orphan_donelist = NULL;
+                rsp->orphan_donetail = &rsp->orphan_donelist;
+        }
+
+        /* And then adopt the callbacks that still need a grace period. */
+        if (rsp->orphan_nxtlist != NULL) {
+                *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
+                rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
+                rsp->orphan_nxtlist = NULL;
+                rsp->orphan_nxttail = &rsp->orphan_nxtlist;
+        }
+}
+
+/*
+ * Trace the fact that this CPU is going offline.
+ */
+static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
+{
+        RCU_TRACE(unsigned long mask);
+        RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda));
+        RCU_TRACE(struct rcu_node *rnp = rdp->mynode);
+
+        RCU_TRACE(mask = rdp->grpmask);
         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(). */
 }
 
 /*
  * The CPU has been completely removed, and some other CPU is reporting
- * this fact from process context.  Do the remainder of the cleanup.
+ * this fact from process context.  Do the remainder of the cleanup,
+ * including orphaning the outgoing CPU's RCU callbacks, and also
+ * adopting them, if there is no _rcu_barrier() instance running.
  * There can only be one CPU hotplug operation at a time, so no other
  * CPU can be attempting to update rcu_cpu_kthread_task.
  */
@@ -1409,17 +1456,21 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
         unsigned long mask;
         int need_report = 0;
         struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
-        struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rnp. */
+        struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
 
         /* Adjust any no-longer-needed kthreads. */
         rcu_stop_cpu_kthread(cpu);
         rcu_node_kthread_setaffinity(rnp, -1);
 
-        /* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
+        /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
 
         /* Exclude any attempts to start a new grace period. */
         raw_spin_lock_irqsave(&rsp->onofflock, flags);
 
+        /* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
+        rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
+        rcu_adopt_orphan_cbs(rsp);
+
         /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
         mask = rdp->grpmask;    /* rnp->grplo is constant. */
         do {
@@ -1456,6 +1507,10 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
 
 #else /* #ifdef CONFIG_HOTPLUG_CPU */
 
+static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
+{
+}
+
 static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
 {
 }
@@ -1524,9 +1579,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
                             rcu_is_callbacks_kthread());
 
         /* Update count, and requeue any remaining callbacks. */
-        rdp->qlen_lazy -= count_lazy;
-        rdp->qlen -= count;
-        rdp->n_cbs_invoked += count;
         if (list != NULL) {
                 *tail = rdp->nxtlist;
                 rdp->nxtlist = list;
@@ -1536,6 +1588,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
                         else
                                 break;
         }
+        smp_mb(); /* List handling before counting for rcu_barrier(). */
+        rdp->qlen_lazy -= count_lazy;
+        rdp->qlen -= count;
+        rdp->n_cbs_invoked += count;
 
         /* Reinstate batch limit if we have worked down the excess. */
         if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
@@ -1824,13 +1880,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
         rdp = this_cpu_ptr(rsp->rda);
 
         /* Add the callback to our list. */
-        *rdp->nxttail[RCU_NEXT_TAIL] = head;
-        rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
         rdp->qlen++;
         if (lazy)
                 rdp->qlen_lazy++;
         else
                 rcu_idle_count_callbacks_posted();
+        smp_mb();  /* Count before adding callback for rcu_barrier(). */
+        *rdp->nxttail[RCU_NEXT_TAIL] = head;
+        rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
 
         if (__is_kfree_rcu_offset((unsigned long)func))
                 trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
@@ -2169,11 +2226,10 @@ static int rcu_cpu_has_callbacks(int cpu)
                rcu_preempt_cpu_has_callbacks(cpu);
 }
 
-static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
-static atomic_t rcu_barrier_cpu_count;
-static DEFINE_MUTEX(rcu_barrier_mutex);
-static struct completion rcu_barrier_completion;
-
+/*
+ * RCU callback function for _rcu_barrier().  If we are last, wake
+ * up the task executing _rcu_barrier().
+ */
 static void rcu_barrier_callback(struct rcu_head *notused)
 {
         if (atomic_dec_and_test(&rcu_barrier_cpu_count))
@@ -2203,27 +2259,94 @@ static void _rcu_barrier(struct rcu_state *rsp,
                          void (*call_rcu_func)(struct rcu_head *head,
                                                void (*func)(struct rcu_head *head)))
 {
-        BUG_ON(in_interrupt());
+        int cpu;
+        unsigned long flags;
+        struct rcu_data *rdp;
+        struct rcu_head rh;
+
+        init_rcu_head_on_stack(&rh);
+
         /* Take mutex to serialize concurrent rcu_barrier() requests. */
         mutex_lock(&rcu_barrier_mutex);
-        init_completion(&rcu_barrier_completion);
+
+        smp_mb();  /* Prevent any prior operations from leaking in. */
+
         /*
-         * Initialize rcu_barrier_cpu_count to 1, then invoke
-         * rcu_barrier_func() on each CPU, so that each CPU also has
-         * incremented rcu_barrier_cpu_count.  Only then is it safe to
-         * decrement rcu_barrier_cpu_count -- otherwise the first CPU
-         * might complete its grace period before all of the other CPUs
-         * did their increment, causing this function to return too
-         * early.  Note that on_each_cpu() disables irqs, which prevents
-         * any CPUs from coming online or going offline until each online
-         * CPU has queued its RCU-barrier callback.
+         * Initialize the count to one rather than to zero in order to
+         * avoid a too-soon return to zero in case of a short grace period
+         * (or preemption of this task).  Also flag this task as doing
+         * an rcu_barrier().  This will prevent anyone else from adopting
+         * orphaned callbacks, which could cause otherwise failure if a
+         * CPU went offline and quickly came back online.  To see this,
+         * consider the following sequence of events:
+         *
+         * 1.   We cause CPU 0 to post an rcu_barrier_callback() callback.
+         * 2.   CPU 1 goes offline, orphaning its callbacks.
+         * 3.   CPU 0 adopts CPU 1's orphaned callbacks.
+         * 4.   CPU 1 comes back online.
+         * 5.   We cause CPU 1 to post an rcu_barrier_callback() callback.
+         * 6.   Both rcu_barrier_callback() callbacks are invoked, awakening
+         *      us -- but before CPU 1's orphaned callbacks are invoked!!!
          */
+        init_completion(&rcu_barrier_completion);
         atomic_set(&rcu_barrier_cpu_count, 1);
-        on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
+        raw_spin_lock_irqsave(&rsp->onofflock, flags);
+        rsp->rcu_barrier_in_progress = current;
+        raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+
+        /*
+         * Force every CPU with callbacks to register a new callback
+         * that will tell us when all the preceding callbacks have
+         * been invoked.  If an offline CPU has callbacks, wait for
+         * it to either come back online or to finish orphaning those
+         * callbacks.
+         */
+        for_each_possible_cpu(cpu) {
+                preempt_disable();
+                rdp = per_cpu_ptr(rsp->rda, cpu);
+                if (cpu_is_offline(cpu)) {
+                        preempt_enable();
+                        while (cpu_is_offline(cpu) && ACCESS_ONCE(rdp->qlen))
+                                schedule_timeout_interruptible(1);
+                } else if (ACCESS_ONCE(rdp->qlen)) {
+                        smp_call_function_single(cpu, rcu_barrier_func,
+                                                 (void *)call_rcu_func, 1);
+                        preempt_enable();
+                } else {
+                        preempt_enable();
+                }
+        }
+
+        /*
+         * Now that all online CPUs have rcu_barrier_callback() callbacks
+         * posted, we can adopt all of the orphaned callbacks and place
+         * an rcu_barrier_callback() callback after them.  When that is done,
+         * we are guaranteed to have an rcu_barrier_callback() callback
+         * following every callback that could possibly have been
+         * registered before _rcu_barrier() was called.
+         */
+        raw_spin_lock_irqsave(&rsp->onofflock, flags);
+        rcu_adopt_orphan_cbs(rsp);
+        rsp->rcu_barrier_in_progress = NULL;
+        raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+        atomic_inc(&rcu_barrier_cpu_count);
+        smp_mb__after_atomic_inc(); /* Ensure atomic_inc() before callback. */
+        call_rcu_func(&rh, rcu_barrier_callback);
+
+        /*
+         * Now that we have an rcu_barrier_callback() callback on each
+         * CPU, and thus each counted, remove the initial count.
+         */
         if (atomic_dec_and_test(&rcu_barrier_cpu_count))
                 complete(&rcu_barrier_completion);
+
+        /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
         wait_for_completion(&rcu_barrier_completion);
+
+        /* Other rcu_barrier() invocations can now safely proceed. */
         mutex_unlock(&rcu_barrier_mutex);
+
+        destroy_rcu_head_on_stack(&rh);
 }
 
 /**
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 36ca28ecedc..1e49c568596 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -371,6 +371,17 @@ struct rcu_state {
 
         raw_spinlock_t onofflock;               /* exclude on/offline and */
                                                 /*  starting new GP. */
+        struct rcu_head *orphan_nxtlist;        /* Orphaned callbacks that */
+                                                /*  need a grace period. */
+        struct rcu_head **orphan_nxttail;       /* Tail of above. */
+        struct rcu_head *orphan_donelist;       /* Orphaned callbacks that */
+                                                /*  are ready to invoke. */
+        struct rcu_head **orphan_donetail;      /* Tail of above. */
+        long qlen_lazy;                         /* Number of lazy callbacks. */
+        long qlen;                              /* Total number of callbacks. */
+        struct task_struct *rcu_barrier_in_progress;
+                                                /* Task doing rcu_barrier(), */
+                                                /*  or NULL if no barrier. */
         raw_spinlock_t fqslock;                 /* Only one task forcing */
                                                 /*  quiescent states. */
         unsigned long jiffies_force_qs;         /* Time at which to invoke */
diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index ed459edeff4..d4bc16ddd1d 100644
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -271,13 +271,13 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
 
         gpnum = rsp->gpnum;
         seq_printf(m, "c=%lu g=%lu s=%d jfq=%ld j=%x "
-                      "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu\n",
+                      "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",
                    rsp->completed, gpnum, rsp->fqs_state,
                    (long)(rsp->jiffies_force_qs - jiffies),
                    (int)(jiffies & 0xffff),
                    rsp->n_force_qs, rsp->n_force_qs_ngp,
                    rsp->n_force_qs - rsp->n_force_qs_ngp,
-                   rsp->n_force_qs_lh);
+                   rsp->n_force_qs_lh, rsp->qlen_lazy, rsp->qlen);
         for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) {
                 if (rnp->level != level) {
                         seq_puts(m, "\n");