rcu: Fix synchronization for rcu_process_gp_end() uses of ->completed counter

Impose a clear locking design on the rcu_process_gp_end()
function's use of the ->completed counter.  This is done by
creating a ->completed field in the rcu_node structure, which
can safely be accessed under the protection of that structure's
lock.  Performance and scalability are maintained by using a
form of double-checked locking, so that rcu_process_gp_end()
only acquires the leaf rcu_node structure's ->lock if a grace
period has recently ended.

This fix reduces rcutorture failure rate by at least two orders
of magnitude under heavy stress with force_quiescent_state()
being invoked artificially often.  Without this fix,
unsynchronized access to the ->completed field can cause
rcu_process_gp_end() to advance callbacks whose grace period has
not yet expired.  (Bad idea!)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
Cc: <stable@kernel.org> # .32.x
LKML-Reference: <12571987494069-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 83 insertions, 48 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 26249abf24dc..9e068d112153 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -570,6 +570,76 @@ check_for_new_grace_period(struct rcu_state *rsp, struct rcu_data *rdp)
 }
 
 /*
+ * Advance this CPU's callbacks, but only if the current grace period
+ * has ended.  This may be called only from the CPU to whom the rdp
+ * belongs.  In addition, the corresponding leaf rcu_node structure's
+ * ->lock must be held by the caller, with irqs disabled.
+ */
+static void
+__rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
+{
+        /* Did another grace period end? */
+        if (rdp->completed != rnp->completed) {
+
+                /* Advance callbacks.  No harm if list empty. */
+                rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
+                rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
+                rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+
+                /* Remember that we saw this grace-period completion. */
+                rdp->completed = rnp->completed;
+        }
+}
+
+/*
+ * Advance this CPU's callbacks, but only if the current grace period
+ * has ended.  This may be called only from the CPU to whom the rdp
+ * belongs.
+ */
+static void
+rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+        unsigned long flags;
+        struct rcu_node *rnp;
+
+        local_irq_save(flags);
+        rnp = rdp->mynode;
+        if (rdp->completed == ACCESS_ONCE(rnp->completed) || /* outside lock. */
+            !spin_trylock(&rnp->lock)) { /* irqs already off, retry later. */
+                local_irq_restore(flags);
+                return;
+        }
+        __rcu_process_gp_end(rsp, rnp, rdp);
+        spin_unlock_irqrestore(&rnp->lock, flags);
+}
+
+/*
+ * Do per-CPU grace-period initialization for running CPU.  The caller
+ * must hold the lock of the leaf rcu_node structure corresponding to
+ * this CPU.
+ */
+static void
+rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
+{
+        /* Prior grace period ended, so advance callbacks for current CPU. */
+        __rcu_process_gp_end(rsp, rnp, rdp);
+
+        /*
+         * Because this CPU just now started the new grace period, we know
+         * that all of its callbacks will be covered by this upcoming grace
+         * period, even the ones that were registered arbitrarily recently.
+         * Therefore, advance all outstanding callbacks to RCU_WAIT_TAIL.
+         *
+         * Other CPUs cannot be sure exactly when the grace period started.
+         * Therefore, their recently registered callbacks must pass through
+         * an additional RCU_NEXT_READY stage, so that they will be handled
+         * by the next RCU grace period.
+         */
+        rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+        rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+}
+
+/*
  * Start a new RCU grace period if warranted, re-initializing the hierarchy
  * 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
@@ -596,26 +666,14 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
         dyntick_record_completed(rsp, rsp->completed - 1);
         note_new_gpnum(rsp, rdp);
 
-        /*
-         * Because this CPU just now started the new grace period, we know
-         * that all of its callbacks will be covered by this upcoming grace
-         * period, even the ones that were registered arbitrarily recently.
-         * Therefore, advance all outstanding callbacks to RCU_WAIT_TAIL.
-         *
-         * Other CPUs cannot be sure exactly when the grace period started.
-         * Therefore, their recently registered callbacks must pass through
-         * an additional RCU_NEXT_READY stage, so that they will be handled
-         * by the next RCU grace period.
-         */
-        rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
-        rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
-
         /* Special-case the common single-level case. */
         if (NUM_RCU_NODES == 1) {
                 rcu_preempt_check_blocked_tasks(rnp);
                 rnp->qsmask = rnp->qsmaskinit;
                 rnp->gpnum = rsp->gpnum;
+                rnp->completed = rsp->completed;
                 rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
+                rcu_start_gp_per_cpu(rsp, rnp, rdp);
                 spin_unlock_irqrestore(&rnp->lock, flags);
                 return;
         }
@@ -648,6 +706,9 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
                 rcu_preempt_check_blocked_tasks(rnp);
                 rnp->qsmask = rnp->qsmaskinit;
                 rnp->gpnum = rsp->gpnum;
+                rnp->completed = rsp->completed;
+                if (rnp == rdp->mynode)
+                        rcu_start_gp_per_cpu(rsp, rnp, rdp);
                 spin_unlock(&rnp->lock);        /* irqs remain disabled. */
         }
 
@@ -659,34 +720,6 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 }
 
 /*
- * Advance this CPU's callbacks, but only if the current grace period
- * has ended.  This may be called only from the CPU to whom the rdp
- * belongs.
- */
-static void
-rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
-{
-        long completed_snap;
-        unsigned long flags;
-
-        local_irq_save(flags);
-        completed_snap = ACCESS_ONCE(rsp->completed);  /* outside of lock. */
-
-        /* Did another grace period end? */
-        if (rdp->completed != completed_snap) {
-
-                /* Advance callbacks.  No harm if list empty. */
-                rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
-                rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
-                rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
-
-                /* Remember that we saw this grace-period completion. */
-                rdp->completed = completed_snap;
-        }
-        local_irq_restore(flags);
-}
-
-/*
  * Clean up after the prior grace period and let rcu_start_gp() start up
  * the next grace period if one is needed.  Note that the caller must
  * hold rnp->lock, as required by rcu_start_gp(), which will release it.
@@ -697,7 +730,6 @@ static void cpu_quiet_msk_finish(struct rcu_state *rsp, unsigned long flags)
         WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
         rsp->completed = rsp->gpnum;
         rsp->signaled = RCU_GP_IDLE;
-        rcu_process_gp_end(rsp, rsp->rda[smp_processor_id()]);
         rcu_start_gp(rsp, flags);  /* releases root node's rnp->lock. */
 }
 
@@ -1539,21 +1571,16 @@ static void __cpuinit
 rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptable)
 {
         unsigned long flags;
-        long lastcomp;
         unsigned long mask;
         struct rcu_data *rdp = rsp->rda[cpu];
         struct rcu_node *rnp = rcu_get_root(rsp);
 
         /* Set up local state, ensuring consistent view of global state. */
         spin_lock_irqsave(&rnp->lock, flags);
-        lastcomp = rsp->completed;
-        rdp->completed = lastcomp;
-        rdp->gpnum = lastcomp;
         rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
         rdp->qs_pending = 1;     /*  so set up to respond to current GP. */
         rdp->beenonline = 1;     /* We have now been online. */
         rdp->preemptable = preemptable;
-        rdp->passed_quiesc_completed = lastcomp - 1;
         rdp->qlen_last_fqs_check = 0;
         rdp->n_force_qs_snap = rsp->n_force_qs;
         rdp->blimit = blimit;
@@ -1575,6 +1602,11 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptable)
                 spin_lock(&rnp->lock);  /* irqs already disabled. */
                 rnp->qsmaskinit |= mask;
                 mask = rnp->grpmask;
+                if (rnp == rdp->mynode) {
+                        rdp->gpnum = rnp->completed; /* if GP in progress... */
+                        rdp->completed = rnp->completed;
+                        rdp->passed_quiesc_completed = rnp->completed - 1;
+                }
                 spin_unlock(&rnp->lock); /* irqs already disabled. */
                 rnp = rnp->parent;
         } while (rnp != NULL && !(rnp->qsmaskinit & mask));
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 8a4c1650ad8d..c1891c3cae63 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -84,6 +84,9 @@ struct rcu_node {
         long    gpnum;          /* Current grace period for this node. */
                                 /*  This will either be equal to or one */
                                 /*  behind the root rcu_node's gpnum. */
+        long    completed;      /* Last grace period completed for this node. */
+                                /*  This will either be equal to or one */
+                                /*  behind the root rcu_node's gpnum. */
         unsigned long qsmask;   /* CPUs or groups that need to switch in */
                                 /*  order for current grace period to proceed.*/
                                 /*  In leaf rcu_node, each bit corresponds to */