kprobes: Replace rp->free_instance with freelist

Gets rid of rp->lock, and as a result kretprobes are now fully
lockless.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/159870623583.1229682.17472357584134058687.stgit@devnote2

include/linux/kprobes.h

@@ -28,6 +28,7 @@
 #include <linux/mutex.h>
 #include <linux/ftrace.h>
 #include <linux/refcount.h>
+#include <linux/freelist.h>
 #include <asm/kprobes.h>
 
 #ifdef CONFIG_KPROBES
@@ -157,17 +158,16 @@ struct kretprobe {
 	int maxactive;
 	int nmissed;
 	size_t data_size;
-	struct hlist_head free_instances;
+	struct freelist_head freelist;
 	struct kretprobe_holder *rph;
-	raw_spinlock_t lock;
 };
 
 struct kretprobe_instance {
 	union {
-		struct llist_node llist;
+		struct freelist_node freelist;
-		struct hlist_node hlist;
 		struct rcu_head rcu;
 	};
+	struct llist_node llist;
 	struct kretprobe_holder *rph;
 	kprobe_opcode_t *ret_addr;
 	void *fp;

kernel/kprobes.c

@@ -1228,11 +1228,8 @@ static void recycle_rp_inst(struct kretprobe_instance *ri)
 {
 	struct kretprobe *rp = get_kretprobe(ri);
 
-	INIT_HLIST_NODE(&ri->hlist);
 	if (likely(rp)) {
-		raw_spin_lock(&rp->lock);
+		freelist_add(&ri->freelist, &rp->freelist);
-		hlist_add_head(&ri->hlist, &rp->free_instances);
-		raw_spin_unlock(&rp->lock);
 	} else
 		call_rcu(&ri->rcu, free_rp_inst_rcu);
 }
@@ -1290,11 +1287,14 @@ NOKPROBE_SYMBOL(kprobe_flush_task);
 static inline void free_rp_inst(struct kretprobe *rp)
 {
 	struct kretprobe_instance *ri;
-	struct hlist_node *next;
+	struct freelist_node *node;
 	int count = 0;
 
-	hlist_for_each_entry_safe(ri, next, &rp->free_instances, hlist) {
+	node = rp->freelist.head;
-		hlist_del(&ri->hlist);
+	while (node) {
+		ri = container_of(node, struct kretprobe_instance, freelist);
+		node = node->next;
+
 		kfree(ri);
 		count++;
 	}
@@ -1925,32 +1925,26 @@ NOKPROBE_SYMBOL(__kretprobe_trampoline_handler)
 static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 {
 	struct kretprobe *rp = container_of(p, struct kretprobe, kp);
-	unsigned long flags = 0;
 	struct kretprobe_instance *ri;
+	struct freelist_node *fn;
 
-	/* TODO: consider to only swap the RA after the last pre_handler fired */
+	fn = freelist_try_get(&rp->freelist);
-	raw_spin_lock_irqsave(&rp->lock, flags);
+	if (!fn) {
-	if (!hlist_empty(&rp->free_instances)) {
-		ri = hlist_entry(rp->free_instances.first,
-				struct kretprobe_instance, hlist);
-		hlist_del(&ri->hlist);
-		raw_spin_unlock_irqrestore(&rp->lock, flags);
-
-		if (rp->entry_handler && rp->entry_handler(ri, regs)) {
-			raw_spin_lock_irqsave(&rp->lock, flags);
-			hlist_add_head(&ri->hlist, &rp->free_instances);
-			raw_spin_unlock_irqrestore(&rp->lock, flags);
-			return 0;
-		}
-
-		arch_prepare_kretprobe(ri, regs);
-
-		__llist_add(&ri->llist, &current->kretprobe_instances);
-
-	} else {
 		rp->nmissed++;
-		raw_spin_unlock_irqrestore(&rp->lock, flags);
+		return 0;
 	}
+
+	ri = container_of(fn, struct kretprobe_instance, freelist);
+
+	if (rp->entry_handler && rp->entry_handler(ri, regs)) {
+		freelist_add(&ri->freelist, &rp->freelist);
+		return 0;
+	}
+
+	arch_prepare_kretprobe(ri, regs);
+
+	__llist_add(&ri->llist, &current->kretprobe_instances);
+
 	return 0;
 }
 NOKPROBE_SYMBOL(pre_handler_kretprobe);
@@ -2007,8 +2001,7 @@ int register_kretprobe(struct kretprobe *rp)
 		rp->maxactive = num_possible_cpus();
 #endif
 	}
-	raw_spin_lock_init(&rp->lock);
+	rp->freelist.head = NULL;
-	INIT_HLIST_HEAD(&rp->free_instances);
 	rp->rph = kzalloc(sizeof(struct kretprobe_holder), GFP_KERNEL);
 	if (!rp->rph)
 		return -ENOMEM;
@@ -2023,8 +2016,7 @@ int register_kretprobe(struct kretprobe *rp)
 			return -ENOMEM;
 		}
 		inst->rph = rp->rph;
-		INIT_HLIST_NODE(&inst->hlist);
+		freelist_add(&inst->freelist, &rp->freelist);
-		hlist_add_head(&inst->hlist, &rp->free_instances);
 	}
 	refcount_set(&rp->rph->ref, i);