Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to allow consolidation

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 07, 2015 at 01:01:20PM +0200, Peter Zijlstra wrote:

> That again doesn't explain which UNLOCKs with non-matching lock values
> it pairs with and what particular ordering is important here.

So after staring at that stuff for a while I came up with the following.
Does this make sense, or am I completely misunderstanding things?

Not been near a compiler.

---
 kernel/rcu/tree.c | 99 ++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 61 insertions(+), 38 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 775d36cc0050..46e1e23ff762 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1460,6 +1460,48 @@ static void trace_rcu_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 }
 
 /*
+ * Wrappers for the rcu_node::lock acquire.
+ *
+ * Because the rcu_nodes form a tree, the tree taversal locking will observe
+ * different lock values, this in turn means that an UNLOCK of one level
+ * followed by a LOCK of another level does not imply a full memory barrier;
+ * and most importantly transitivity is lost.
+ *
+ * In order to restore full ordering between tree levels, augment the regular
+ * lock acquire functions with smp_mb__after_unlock_lock().
+ */
+static inline void raw_spin_lock_rcu_node(struct rcu_node *rnp)
+{
+	raw_spin_lock(&rnp->lock);
+	smp_mb__after_unlock_lock();
+}
+
+static inline void raw_spin_lock_irq_rcu_node(struct rcu_node *rnp)
+{
+	raw_spin_lock_irq(&rnp->lock);
+	smp_mb__after_unlock_lock();
+}
+
+static inline void
+_raw_spin_lock_irqsave_rcu_node(struct rcu_node *rnp, unsigned long *flags)
+{
+	_raw_spin_lock_irqsave(&rnp->lock, flags);
+	smp_mb__after_unlock_lock();
+}
+
+#define raw_spin_lock_irqsave_rcu_node(rnp, flags)
+	_raw_spin_lock_irqsave_rcu_node((rnp), &(flags))
+
+static inline bool raw_spin_trylock_rcu_node(struct rcu_node *rnp)
+{
+	bool locked = raw_spin_trylock(&rnp->lock);
+	if (locked)
+		smp_mb__after_unlock_lock();
+	return locked;
+}
+
+
+/*
  * Start some future grace period, as needed to handle newly arrived
  * callbacks.  The required future grace periods are recorded in each
  * rcu_node structure's ->need_future_gp field.  Returns true if there
@@ -1512,10 +1554,8 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	 * hold it, acquire the root rcu_node structure's lock in order to
 	 * start one (if needed).
 	 */
-	if (rnp != rnp_root) {
-		raw_spin_lock(&rnp_root->lock);
-		smp_mb__after_unlock_lock();
-	}
+	if (rnp != rnp_root)
+		raw_spin_lock_rcu_node(rnp);
 
 	/*
 	 * Get a new grace-period number.  If there really is no grace
@@ -1764,11 +1804,10 @@ static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
 	if ((rdp->gpnum == READ_ONCE(rnp->gpnum) &&
 	     rdp->completed == READ_ONCE(rnp->completed) &&
 	     !unlikely(READ_ONCE(rdp->gpwrap))) || /* w/out lock. */
-	    !raw_spin_trylock(&rnp->lock)) { /* irqs already off, so later. */
+	    !raw_spin_trylock_rcu_node(rnp)) { /* irqs already off, so later. */
 		local_irq_restore(flags);
 		return;
 	}
-	smp_mb__after_unlock_lock();
 	needwake = __note_gp_changes(rsp, rnp, rdp);
 	raw_spin_unlock_irqrestore(&rnp->lock, flags);
 	if (needwake)
@@ -1792,8 +1831,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	WRITE_ONCE(rsp->gp_activity, jiffies);
-	raw_spin_lock_irq(&rnp->lock);
-	smp_mb__after_unlock_lock();
+	raw_spin_lock_irq_rcu_node(rnp);
 	if (!READ_ONCE(rsp->gp_flags)) {
 		/* Spurious wakeup, tell caller to go back to sleep.  */
 		raw_spin_unlock_irq(&rnp->lock);
@@ -1825,8 +1863,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
 	 */
 	rcu_for_each_leaf_node(rsp, rnp) {
 		rcu_gp_slow(rsp, gp_preinit_delay);
-		raw_spin_lock_irq(&rnp->lock);
-		smp_mb__after_unlock_lock();
+		raw_spin_lock_irq_rcu_node(rnp);
 		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
 		    !rnp->wait_blkd_tasks) {
 			/* Nothing to do on this leaf rcu_node structure. */
@@ -1882,8 +1919,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
 	 */
 	rcu_for_each_node_breadth_first(rsp, rnp) {
 		rcu_gp_slow(rsp, gp_init_delay);
-		raw_spin_lock_irq(&rnp->lock);
-		smp_mb__after_unlock_lock();
+		raw_spin_lock_irq_rcu_node(rnp);
 		rdp = this_cpu_ptr(rsp->rda);
 		rcu_preempt_check_blocked_tasks(rnp);
 		rnp->qsmask = rnp->qsmaskinit;
@@ -1953,8 +1989,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 	}
 	/* Clear flag to prevent immediate re-entry. */
 	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
-		raw_spin_lock_irq(&rnp->lock);
-		smp_mb__after_unlock_lock();
+		raw_spin_lock_irq_rcu_node(rnp);
 		WRITE_ONCE(rsp->gp_flags,
 			   READ_ONCE(rsp->gp_flags) & ~RCU_GP_FLAG_FQS);
 		raw_spin_unlock_irq(&rnp->lock);
@@ -1974,8 +2009,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	WRITE_ONCE(rsp->gp_activity, jiffies);
-	raw_spin_lock_irq(&rnp->lock);
-	smp_mb__after_unlock_lock();
+	raw_spin_lock_irq_rcu_node(rnp);
 	gp_duration = jiffies - rsp->gp_start;
 	if (gp_duration > rsp->gp_max)
 		rsp->gp_max = gp_duration;
@@ -2000,8 +2034,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	 * grace period is recorded in any of the rcu_node structures.
 	 */
 	rcu_for_each_node_breadth_first(rsp, rnp) {
-		raw_spin_lock_irq(&rnp->lock);
-		smp_mb__after_unlock_lock();
+		raw_spin_lock_irq_rcu_node(rnp);
 		WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
 		WARN_ON_ONCE(rnp->qsmask);
 		WRITE_ONCE(rnp->completed, rsp->gpnum);
@@ -2016,8 +2049,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 		rcu_gp_slow(rsp, gp_cleanup_delay);
 	}
 	rnp = rcu_get_root(rsp);
-	raw_spin_lock_irq(&rnp->lock);
-	smp_mb__after_unlock_lock(); /* Order GP before ->completed update. */
+	raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
 	rcu_nocb_gp_set(rnp, nocb);
 
 	/* Declare grace period done. */
@@ -2264,8 +2296,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
 		raw_spin_unlock_irqrestore(&rnp->lock, flags);
 		rnp_c = rnp;
 		rnp = rnp->parent;
-		raw_spin_lock_irqsave(&rnp->lock, flags);
-		smp_mb__after_unlock_lock();
+		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		oldmask = rnp_c->qsmask;
 	}
 
@@ -2312,8 +2343,7 @@ static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
 	gps = rnp->gpnum;
 	mask = rnp->grpmask;
 	raw_spin_unlock(&rnp->lock);	/* irqs remain disabled. */
-	raw_spin_lock(&rnp_p->lock);	/* irqs already disabled. */
-	smp_mb__after_unlock_lock();
+	raw_spin_lock_rcu_node(rnp);	/* irqs already disabled. */
 	rcu_report_qs_rnp(mask, rsp, rnp_p, gps, flags);
 }
 
@@ -2335,8 +2365,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
 	struct rcu_node *rnp;
 
 	rnp = rdp->mynode;
-	raw_spin_lock_irqsave(&rnp->lock, flags);
-	smp_mb__after_unlock_lock();
+	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	if ((rdp->passed_quiesce == 0 &&
 	     rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr)) ||
 	    rdp->gpnum != rnp->gpnum || rnp->completed == rnp->gpnum ||
@@ -2562,8 +2591,7 @@ static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf)
 		rnp = rnp->parent;
 		if (!rnp)
 			break;
-		raw_spin_lock(&rnp->lock); /* irqs already disabled. */
-		smp_mb__after_unlock_lock(); /* GP memory ordering. */
+		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
 		rnp->qsmaskinit &= ~mask;
 		rnp->qsmask &= ~mask;
 		if (rnp->qsmaskinit) {
@@ -2591,8 +2619,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp)
 
 	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
 	mask = rdp->grpmask;
-	raw_spin_lock_irqsave(&rnp->lock, flags);
-	smp_mb__after_unlock_lock();	/* Enforce GP memory-order guarantee. */
+	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */
 	rnp->qsmaskinitnext &= ~mask;
 	raw_spin_unlock_irqrestore(&rnp->lock, flags);
 }
@@ -2789,8 +2816,7 @@ static void force_qs_rnp(struct rcu_state *rsp,
 	rcu_for_each_leaf_node(rsp, rnp) {
 		cond_resched_rcu_qs();
 		mask = 0;
-		raw_spin_lock_irqsave(&rnp->lock, flags);
-		smp_mb__after_unlock_lock();
+		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		if (rnp->qsmask == 0) {
 			if (rcu_state_p == &rcu_sched_state ||
 			    rsp != rcu_state_p ||
@@ -2861,8 +2887,7 @@ static void force_quiescent_state(struct rcu_state *rsp)
 	/* rnp_old == rcu_get_root(rsp), rnp == NULL. */
 
 	/* Reached the root of the rcu_node tree, acquire lock. */
-	raw_spin_lock_irqsave(&rnp_old->lock, flags);
-	smp_mb__after_unlock_lock();
+	raw_spin_lock_irqsave_rcu_node(rnp_old, flags);
 	raw_spin_unlock(&rnp_old->fqslock);
 	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
 		rsp->n_force_qs_lh++;
@@ -2985,8 +3010,7 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
 		if (!rcu_gp_in_progress(rsp)) {
 			struct rcu_node *rnp_root = rcu_get_root(rsp);
 
-			raw_spin_lock(&rnp_root->lock);
-			smp_mb__after_unlock_lock();
+			raw_spin_lock_rcu_node(rnp_root);
 			needwake = rcu_start_gp(rsp);
 			raw_spin_unlock(&rnp_root->lock);
 			if (needwake)
@@ -3925,8 +3949,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	 */
 	rnp = rdp->mynode;
 	mask = rdp->grpmask;
-	raw_spin_lock(&rnp->lock);		/* irqs already disabled. */
-	smp_mb__after_unlock_lock();
+	raw_spin_lock_rcu_node(rnp);		/* irqs already disabled. */
 	rnp->qsmaskinitnext |= mask;
 	rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
 	rdp->completed = rnp->completed;