srcu: expose srcu_readers_active()

This is needed by rust/helpers/srcu.c which now adds
rust_helper_srcu_readers_active() as a wrapper around the SRCU helper
for Rust callers.

To achive this:

1- Move the srcu_readers_active() implementation from
   "kernel/rcu/srcutree.c" to "include/linux/srcutree.h".

2- Implement a matching srcu_readers_active() in
   "include/linux/srcutiny.h" and use it on the existing open-coded
   WARN_ON() check in cleanup_srcu_struct().

Signed-off-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/srcutiny.h

@@ -154,4 +154,17 @@ static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
 		 data_race(READ_ONCE(ssp->srcu_idx_max)));
 }
 
+/**
+ * srcu_readers_active - returns true if there are readers. and false otherwise.
+ * @ssp: which srcu_struct to count active readers (holding srcu_read_lock).
+ *
+ * Note that this is not an atomic primitive, and can therefore suffer
+ * severe errors when invoked on an active srcu_struct. That said, it
+ * can be useful as an error check at cleanup time.
+ */
+static inline bool srcu_readers_active(struct srcu_struct *ssp)
+{
+	return READ_ONCE(ssp->srcu_lock_nesting[0]) || READ_ONCE(ssp->srcu_lock_nesting[1]);
+}
+
 #endif

include/linux/srcutree.h

@@ -374,4 +374,28 @@ static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flav
 		__srcu_check_read_flavor(ssp, read_flavor);
 }
 
+/**
+ * srcu_readers_active - returns true if there are readers. and false otherwise.
+ * @ssp: which srcu_struct to count active readers (holding srcu_read_lock).
+ *
+ * Note that this is not an atomic primitive, and can therefore suffer
+ * severe errors when invoked on an active srcu_struct. That said, it
+ * can be useful as an error check at cleanup time.
+ */
+static inline bool srcu_readers_active(struct srcu_struct *ssp)
+{
+	int cpu;
+	unsigned long sum = 0;
+
+	for_each_possible_cpu(cpu) {
+		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
+
+		sum += atomic_long_read(&sdp->srcu_ctrs[0].srcu_locks);
+		sum += atomic_long_read(&sdp->srcu_ctrs[1].srcu_locks);
+		sum -= atomic_long_read(&sdp->srcu_ctrs[0].srcu_unlocks);
+		sum -= atomic_long_read(&sdp->srcu_ctrs[1].srcu_unlocks);
+	}
+	return sum;
+}
+
 #endif

kernel/rcu/srcutiny.c

@@ -85,7 +85,7 @@ EXPORT_SYMBOL_GPL(init_srcu_struct_generic);
  */
 void cleanup_srcu_struct(struct srcu_struct *ssp)
 {
-	WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
+	WARN_ON(srcu_readers_active(ssp));
 	irq_work_sync(&ssp->srcu_irq_work);
 	flush_work(&ssp->srcu_work);
 	WARN_ON(ssp->srcu_gp_running);

kernel/rcu/srcutree.c

@@ -599,31 +599,6 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
 	return srcu_readers_lock_idx(ssp, idx, did_gp, unlocks);
 }
 
-/**
- * srcu_readers_active - returns true if there are readers. and false
- *                       otherwise
- * @ssp: which srcu_struct to count active readers (holding srcu_read_lock).
- *
- * Note that this is not an atomic primitive, and can therefore suffer
- * severe errors when invoked on an active srcu_struct.  That said, it
- * can be useful as an error check at cleanup time.
- */
-static bool srcu_readers_active(struct srcu_struct *ssp)
-{
-	int cpu;
-	unsigned long sum = 0;
-
-	for_each_possible_cpu(cpu) {
-		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
-
-		sum += atomic_long_read(&sdp->srcu_ctrs[0].srcu_locks);
-		sum += atomic_long_read(&sdp->srcu_ctrs[1].srcu_locks);
-		sum -= atomic_long_read(&sdp->srcu_ctrs[0].srcu_unlocks);
-		sum -= atomic_long_read(&sdp->srcu_ctrs[1].srcu_unlocks);
-	}
-	return sum;
-}
-
 /*
  * We use an adaptive strategy for synchronize_srcu() and especially for
  * synchronize_srcu_expedited().  We spin for a fixed time period

rust/helpers/srcu.c

@@ -9,6 +9,11 @@ __rust_helper int rust_helper_init_srcu_struct_with_key(struct srcu_struct *ssp,
 	return __init_srcu_struct(ssp, name, key);
 }
 
+__rust_helper bool rust_helper_srcu_readers_active(struct srcu_struct *ssp)
+{
+	return srcu_readers_active(ssp);
+}
+
 __rust_helper int rust_helper_srcu_read_lock(struct srcu_struct *ssp)
 {
 	return srcu_read_lock(ssp);