[PATCH 4/6] srcu: Make Tiny call_srcu() safe to call from any context

[Puranjay Mohan]

Make Tiny call_srcu() safe to call from unknown context: when interrupts
are disabled and the scheduler is up, stage the callback on the
srcu_struct's lockless list that an irq_work re-issues later.  Tiny SRCU
is uniprocessor, so there is no CPU-offline drain.  Gated by
CONFIG_RCU_DEFER, like Tree SRCU.

Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
---
 include/linux/srcutiny.h | 11 ++++++---
 kernel/rcu/srcutiny.c    | 53 +++++++++++++++++++++++++++++++++++++---
 2 files changed, 56 insertions(+), 8 deletions(-)

diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index fbcf13bc12d15..47275d182966c 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -12,6 +12,7 @@
 #define _LINUX_SRCU_TINY_H
 
 #include <linux/irq_work_types.h>
+#include <linux/llist.h>
 #include <linux/swait.h>
 
 struct srcu_struct {
@@ -26,6 +27,8 @@ struct srcu_struct {
 	struct rcu_head **srcu_cb_tail;	/* Pending callbacks: Tail. */
 	struct work_struct srcu_work;	/* For driving grace periods. */
 	struct irq_work srcu_irq_work;	/* Defer schedule_work() to irq work. */
+	struct llist_head defer_cbs;	/* Callbacks deferred on re-entry. */
+	struct irq_work defer_iw;		/* Registers defer_cbs later. */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
@@ -33,6 +36,7 @@ struct srcu_struct {
 
 void srcu_drive_gp(struct work_struct *wp);
 void srcu_tiny_irq_work(struct irq_work *irq_work);
+void srcu_defer_drain(struct irq_work *irq_work);
 
 #define __SRCU_STRUCT_INIT(name, __ignored, ___ignored, ____ignored)	\
 {									\
@@ -40,6 +44,8 @@ void srcu_tiny_irq_work(struct irq_work *irq_work);
 	.srcu_cb_tail = &name.srcu_cb_head,				\
 	.srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp),	\
 	.srcu_irq_work = { .func = srcu_tiny_irq_work },		\
+	.defer_cbs = LLIST_HEAD_INIT(name.defer_cbs),			\
+	.defer_iw = { .func = srcu_defer_drain },				\
 	__SRCU_DEP_MAP_INIT(name)					\
 }
 
@@ -131,10 +137,7 @@ static inline void synchronize_srcu_expedited(struct srcu_struct *ssp)
 	synchronize_srcu(ssp);
 }
 
-static inline void srcu_barrier(struct srcu_struct *ssp)
-{
-	synchronize_srcu(ssp);
-}
+void srcu_barrier(struct srcu_struct *ssp);
 
 static inline void srcu_expedite_current(struct srcu_struct *ssp) { }
 #define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index f9c498ae75df2..b454d3e828edc 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -10,6 +10,7 @@
 
 #include <linux/export.h>
 #include <linux/irq_work.h>
+#include <linux/llist.h>
 #include <linux/mutex.h>
 #include <linux/preempt.h>
 #include <linux/rcupdate_wait.h>
@@ -43,6 +44,8 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
 	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
 	INIT_LIST_HEAD(&ssp->srcu_work.entry);
 	init_irq_work(&ssp->srcu_irq_work, srcu_tiny_irq_work);
+	init_llist_head(&ssp->defer_cbs);
+	init_irq_work(&ssp->defer_iw, srcu_defer_drain);
 	return 0;
 }
 
@@ -215,11 +218,11 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
 }
 
 /*
- * Enqueue an SRCU callback on the specified srcu_struct structure,
- * initiating grace-period processing if it is not already running.
+ * Enqueue @rhp on the callback list.  Also called by srcu_defer_drain() to
+ * re-issue a deferred callback, so it must not re-check the deferral condition.
  */
-void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
-	       rcu_callback_t func)
+static void srcu_do_enqueue(struct srcu_struct *ssp, struct rcu_head *rhp,
+			    rcu_callback_t func)
 {
 	unsigned long flags;
 
@@ -233,6 +236,39 @@ void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 	srcu_gp_start_if_needed(ssp);
 	preempt_enable();
 }
+
+void srcu_defer_drain(struct irq_work *iw)
+{
+	struct srcu_struct *ssp = container_of(iw, struct srcu_struct, defer_iw);
+	struct llist_node *node, *next;
+
+	/*
+	 * Callbacks have no mutual ordering guarantee and srcu_barrier() has
+	 * already flushed us, so drain in llist order without reversing.
+	 */
+	llist_for_each_safe(node, next, llist_del_all(&ssp->defer_cbs)) {
+		struct rcu_head *rhp = (struct rcu_head *)node;
+
+		srcu_do_enqueue(ssp, rhp, rhp->func);
+	}
+}
+EXPORT_SYMBOL_GPL(srcu_defer_drain);
+
+void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
+	       rcu_callback_t func)
+{
+	if (should_rcu_defer()) {
+		rhp->func = func;
+		if (llist_add((struct llist_node *)rhp, &ssp->defer_cbs))
+			irq_work_queue(&ssp->defer_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);
+}
 EXPORT_SYMBOL_GPL(call_srcu);
 
 /*
@@ -262,6 +298,15 @@ void synchronize_srcu(struct srcu_struct *ssp)
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
+/* Register any deferred callbacks, then wait for all in-flight ones. */
+void srcu_barrier(struct srcu_struct *ssp)
+{
+	if (IS_ENABLED(CONFIG_RCU_DEFER))
+		irq_work_sync(&ssp->defer_iw);
+	synchronize_srcu(ssp);
+}
+EXPORT_SYMBOL_GPL(srcu_barrier);
+
 /*
  * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
  */
-- 
2.53.0-Meta