srcu: Prevent cleanup_srcu_struct() from freeing non-dynamic ->sda

When an srcu_struct structure is created (but not in a kernel module)
by DEFINE_SRCU() and friends, the per-CPU srcu_data structure is
statically allocated.  In all other cases, that structure is obtained
from alloc_percpu(), in which case cleanup_srcu_struct() must invoke
free_percpu() on the resulting ->sda pointer in the srcu_struct pointer.

Which it does.

Except that it also invokes free_percpu() on the ->sda pointer
referencing the statically allocated per-CPU srcu_data structures.
Which free_percpu() is surprisingly OK with.

This commit nevertheless stops cleanup_srcu_struct() from freeing
statically allocated per-CPU srcu_data structures.
Co-developed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/srcutree.h

@@ -73,6 +73,7 @@ struct srcu_struct {
 	unsigned long srcu_gp_seq_needed_exp;	/* Furthest future exp GP. */
 	unsigned long srcu_last_gp_end;		/* Last GP end timestamp (ns) */
 	struct srcu_data __percpu *sda;		/* Per-CPU srcu_data array. */
+	bool sda_is_static;			/* May ->sda be passed to free_percpu()? */
 	unsigned long srcu_barrier_seq;		/* srcu_barrier seq #. */
 	struct mutex srcu_barrier_mutex;	/* Serialize barrier ops. */
 	struct completion srcu_barrier_completion;

kernel/rcu/srcutree.c

@@ -217,6 +217,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	mutex_init(&ssp->srcu_barrier_mutex);
 	atomic_set(&ssp->srcu_barrier_cpu_cnt, 0);
 	INIT_DELAYED_WORK(&ssp->work, process_srcu);
+	ssp->sda_is_static = is_static;
 	if (!is_static)
 		ssp->sda = alloc_percpu(struct srcu_data);
 	if (!ssp->sda)
@@ -226,7 +227,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	if (READ_ONCE(ssp->srcu_size_state) == SRCU_SIZE_SMALL && convert_to_big == 1) {
 		if (!init_srcu_struct_nodes(ssp, GFP_ATOMIC)) {
-			if (!is_static) {
+			if (!ssp->sda_is_static) {
 				free_percpu(ssp->sda);
 				ssp->sda = NULL;
 				return -ENOMEM;
@@ -446,8 +447,10 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
 			rcu_seq_current(&ssp->srcu_gp_seq), ssp->srcu_gp_seq_needed);
 		return; /* Caller forgot to stop doing call_srcu()? */
 	}
-	free_percpu(ssp->sda);
-	ssp->sda = NULL;
+	if (!ssp->sda_is_static) {
+		free_percpu(ssp->sda);
+		ssp->sda = NULL;
+	}
 	kfree(ssp->node);
 	ssp->node = NULL;
 	ssp->srcu_size_state = SRCU_SIZE_SMALL;