[PATCH 3/6] srcu: Make call_srcu() safe to call from any context

[Puranjay Mohan <puranjay@kernel.org>]

call_srcu() has the same constraint as call_rcu(): its callback list and
locks are only touched with interrupts disabled.  srcu_gp_start_if_needed()
enqueues under raw_spin_lock_irqsave() and may walk the srcu_node tree,
and callback invocation and grace-period work do likewise.  So a
call_srcu() with interrupts disabled may be racing a list operation in
flight on this CPU, and enqueuing directly could corrupt the list or
deadlock.

Defer as call_rcu() does: stage the callback on the srcu_data's
->defer_cbs, chain that srcu_data onto a per-CPU list, and kick a per-CPU
irq_work that re-issues it straight to the enqueue helper -- never back
through __call_srcu(), so it cannot defer again.

The irq_work is global (per CPU, not per srcu_struct) and statically
initialized, so the deferral never runs check_init_srcu_struct().  It is
IRQ_WORK_INIT_HARD to re-issue promptly, as for call_rcu().  srcu_barrier()
and cleanup_srcu_struct() flush it first, and rcutree_migrate_callbacks()
calls srcu_offline_drain() to drain an outgoing CPU's deferred callbacks.
Gated by CONFIG_RCU_DEFER, as for call_rcu().

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 include/linux/srcutree.h |   4 ++
 kernel/rcu/rcu.h         |   3 +
 kernel/rcu/srcutree.c    | 138 ++++++++++++++++++++++++++++++++++++++-
 kernel/rcu/tree.c        |   2 +
 4 files changed, 144 insertions(+), 3 deletions(-)

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 75e54e4f963fa..1ce759fb70948 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -13,6 +13,8 @@
 
 #include <linux/rcu_node_tree.h>
 #include <linux/completion.h>
+#include <linux/irq_work_types.h>
+#include <linux/llist.h>
 
 struct srcu_node;
 struct srcu_struct;
@@ -41,6 +43,8 @@ struct srcu_data {
 	bool srcu_cblist_invoking;		/* Invoking these CBs? */
 	struct timer_list delay_work;		/* Delay for CB invoking */
 	struct work_struct work;		/* Context for CB invoking. */
+	struct llist_head defer_cbs;		/* Callbacks deferred on re-entry. */
+	struct llist_node defer_link;		/* Links onto the per-CPU deferral drain list */
 	struct rcu_head srcu_barrier_head;	/* For srcu_barrier() use. */
 	struct rcu_head srcu_ec_head;		/* For srcu_expedite_current() use. */
 	int srcu_ec_state;			/*  State for srcu_expedite_current(). */
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index ed6604445f2ac..036d28516e92d 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -587,6 +587,9 @@ static inline bool should_rcu_defer(void)
 	return irqs_disabled() && rcu_scheduler_active != RCU_SCHEDULER_INACTIVE;
 }
 
+/* Drain an outgoing CPU's deferred SRCU callbacks; see rcutree_migrate_callbacks(). */
+void srcu_offline_drain(int cpu);
+
 enum rcutorture_type {
 	RCU_FLAVOR,
 	RCU_TASKS_FLAVOR,
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 304112674e8a2..b9fe57ff91000 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -20,6 +20,7 @@
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/irq_work.h>
+#include <linux/llist.h>
 #include <linux/rcupdate_wait.h>
 #include <linux/sched.h>
 #include <linux/smp.h>
@@ -79,6 +80,46 @@ static void process_srcu(struct work_struct *work);
 static void srcu_irq_work(struct irq_work *work);
 static void srcu_delay_timer(struct timer_list *t);
 
+static void srcu_defer_drain(struct irq_work *iw);
+
+/*
+ * Per-CPU call_srcu() deferral state.  The irq_work is global (shared by every
+ * srcu_struct), so it must find which srcu_structs deferred callbacks: a
+ * callback is staged on its srcu_data's ->defer_cbs, and that srcu_data is
+ * chained (->defer_link) onto this per-CPU ->list that the irq_work walks.
+ */
+struct srcu_defer {
+	struct llist_head	list;
+	struct irq_work		iw;
+	raw_spinlock_t		lock;
+};
+
+static DEFINE_PER_CPU(struct srcu_defer, srcu_defer) = {
+	.lock = __RAW_SPIN_LOCK_UNLOCKED(srcu_defer.lock),
+	.iw = IRQ_WORK_INIT_HARD(srcu_defer_drain),
+};
+
+/*
+ * Register pending deferred call_srcu() callbacks so a following srcu_barrier()
+ * waits for them.  An online CPU's own irq_work re-issues its callbacks, so wait
+ * it out; an offline CPU's irq_work may never run again, so drain its list
+ * directly onto this CPU instead.
+ */
+static void srcu_defer_flush(void)
+{
+	int cpu;
+
+	if (!IS_ENABLED(CONFIG_RCU_DEFER))
+		return;
+
+	for_each_possible_cpu(cpu) {
+		if (cpu_online(cpu))
+			irq_work_sync(&per_cpu(srcu_defer, cpu).iw);
+		else
+			srcu_defer_drain(&per_cpu(srcu_defer, cpu).iw);
+	}
+}
+
 /*
  * Initialize SRCU per-CPU data.  Note that statically allocated
  * srcu_struct structures might already have srcu_read_lock() and
@@ -107,6 +148,11 @@ static void init_srcu_struct_data(struct srcu_struct *ssp)
 		sdp->cpu = cpu;
 		INIT_WORK(&sdp->work, srcu_invoke_callbacks);
 		timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
+		/*
+		 * ->defer_cbs and ->defer_link are valid when zeroed and are not
+		 * reinitialized here, lest we clobber callbacks a reentrant
+		 * call_srcu() already staged.  See __call_srcu().
+		 */
 		sdp->ssp = ssp;
 	}
 }
@@ -697,6 +743,8 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
 		return; /* Just leak it! */
 	/* Wait for irq_work to finish first as it may queue a new work. */
 	irq_work_sync(&sup->irq_work);
+	/* Drain any deferred callbacks (shared irq_work) before freeing ->sda. */
+	srcu_defer_flush();
 	flush_delayed_work(&sup->work);
 	for_each_possible_cpu(cpu) {
 		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
@@ -1410,8 +1458,18 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
  * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
  * srcu_struct structure.
  */
-static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
-			rcu_callback_t func, bool do_norm)
+/*
+ * The srcu_cblist and the srcu_node tree are only ever accessed with interrupts
+ * disabled (srcu_gp_start_if_needed() enqueues under raw_spin_lock_irqsave() and
+ * may walk the tree; callback invocation and grace-period work likewise).  So
+ * like call_rcu(), enqueuing while a list operation is in flight on this CPU
+ * would corrupt it or deadlock -- e.g. call_rcu_tasks_trace(), which is
+ * call_srcu() on a dedicated srcu_struct, from a BPF program.  __call_srcu()
+ * defers whenever interrupts are disabled, via a per-CPU irq_work; only kernels
+ * where that can happen (CONFIG_RCU_DEFER) need this.
+ */
+static void srcu_do_enqueue(struct srcu_struct *ssp, struct rcu_head *rhp,
+			    rcu_callback_t func, bool do_norm)
 {
 	if (debug_rcu_head_queue(rhp)) {
 		/* Probable double call_srcu(), so leak the callback. */
@@ -1423,6 +1481,72 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 	(void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
 }
 
+static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
+			rcu_callback_t func, bool do_norm)
+{
+	if (should_rcu_defer()) {
+		struct srcu_data *sdp = this_cpu_ptr(ssp->sda);
+
+		rhp->func = func;
+		if (llist_add((struct llist_node *)rhp, &sdp->defer_cbs)) {
+			/* First deferral on this srcu_data: chain it for the drain. */
+			struct srcu_defer *sndp = this_cpu_ptr(&srcu_defer);
+
+			sdp->ssp = ssp;
+			if (llist_add(&sdp->defer_link, &sndp->list))
+				irq_work_queue(&sndp->iw);
+		}
+		return;
+	}
+
+	/* An NMI reaching here entered with irqs enabled, so the enqueue can race. */
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
+
+	srcu_do_enqueue(ssp, rhp, func, do_norm);
+}
+
+/*
+ * Re-issue this CPU's deferred callbacks, going straight to srcu_do_enqueue() so
+ * they cannot defer again.  The irq_work walks its ->list of srcu_data, each
+ * carrying its own ->defer_cbs.  ->lock is held across the drain so drainers
+ * (irq_work, srcu_defer_flush() and srcu_offline_drain()) serialize and never
+ * leave a callback off the lists yet not on a callback list.
+ */
+static void srcu_defer_drain(struct irq_work *iw)
+{
+	struct srcu_defer *sndp = container_of(iw, struct srcu_defer, iw);
+	struct llist_node *snode, *snext;
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&sndp->lock, flags);
+	llist_for_each_safe(snode, snext, llist_del_all(&sndp->list)) {
+		struct srcu_data *sdp = container_of(snode, struct srcu_data, defer_link);
+		struct srcu_struct *ssp = sdp->ssp;
+		struct llist_node *cnode, *cnext;
+
+		cnode = llist_del_all(&sdp->defer_cbs);
+		llist_for_each_safe(cnode, cnext, cnode) {
+			struct rcu_head *rhp = (struct rcu_head *)cnode;
+
+			srcu_do_enqueue(ssp, rhp, rhp->func, true);
+		}
+	}
+	raw_spin_unlock_irqrestore(&sndp->lock, flags);
+}
+
+/*
+ * Drain @cpu's deferred call_srcu() callbacks; called from
+ * rcutree_migrate_callbacks() once @cpu is dead.  One pass covers every
+ * srcu_struct, as the per-CPU srcu_defer list chains them all, and the
+ * re-issue lands on the current CPU's srcu_data.
+ */
+void srcu_offline_drain(int cpu)
+{
+	if (!IS_ENABLED(CONFIG_RCU_DEFER))
+		return;
+	srcu_defer_drain(&per_cpu(srcu_defer, cpu).iw);
+}
+
 /**
  * call_srcu() - Queue a callback for invocation after an SRCU grace period
  * @ssp: srcu_struct in queue the callback
@@ -1677,9 +1801,17 @@ void srcu_barrier(struct srcu_struct *ssp)
 {
 	int cpu;
 	int idx;
-	unsigned long s = rcu_seq_snap(&ssp->srcu_sup->srcu_barrier_seq);
+	unsigned long s;
 
 	check_init_srcu_struct(ssp);
+
+	/*
+	 * Register any deferred callbacks before snapshotting the sequence.  The
+	 * shared irq_work may also drain other srcu_structs', which is harmless.
+	 */
+	srcu_defer_flush();
+
+	s = rcu_seq_snap(&ssp->srcu_sup->srcu_barrier_seq);
 	mutex_lock(&ssp->srcu_sup->srcu_barrier_mutex);
 	if (rcu_seq_done(&ssp->srcu_sup->srcu_barrier_seq, s)) {
 		smp_mb(); /* Force ordering following return. */
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 93c4242345e67..f8c6930041746 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4627,6 +4627,8 @@ void rcutree_migrate_callbacks(int cpu)
 	 * returns; the re-issue lands on this CPU.
 	 */
 	rcu_defer_drain(&rdp->defer_work);
+	/* Likewise for the outgoing CPU's deferred call_srcu() callbacks. */
+	srcu_offline_drain(cpu);
 
 	if (rcu_rdp_is_offloaded(rdp))
 		return;
-- 
2.53.0-Meta