[PATCH slab/for-next v4 7/8] mm/slab: introduce kfree_rcu_nolock()

[Harry Yoo (Oracle)]

Currently, k[v]free_rcu() cannot be called in unknown context since
it could lead to a deadlock when called in the middle of k[v]free_rcu().

Make users' lives easier by introducing kfree_rcu_nolock() variant,
now that kfree_rcu_sheaf() is available on PREEMPT_RT and
__kfree_rcu_sheaf() handles unknown context.

kfree_rcu_nolock() falls back to the kvfree_rcu batching when sheaves
path fails. In most cases, the sheaves path is expected to succeed
and it's unnecessary to add complexity to the existing kvfree_rcu
batching by teaching it how to handle unknown context.

Since defer_kfree_rcu() can be called on caches without sheaves, move
deferred_work_barrier() and rcu_barrier() outside the branch in
kvfree_rcu_barrier_on_cache().

Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
---
 include/linux/rcupdate.h | 19 +++++++++++++++++++
 mm/slab_common.c         | 28 ++++++++++++++++++++++++++--
 mm/slub.c                | 24 +++++++++++++++++++++++-
 3 files changed, 68 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index ef5bb6981133..ce0fd401352e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -1099,6 +1099,7 @@ static inline void rcu_read_unlock_migrate(void)
  * In mm/slab_common.c, no suitable header to include here.
  */
 void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr);
+void kfree_call_rcu_nolock(struct kvfree_rcu_head *head, void *ptr);
 
 /*
  * The BUILD_BUG_ON() makes sure the rcu_head offset can be handled. See the
@@ -1124,6 +1125,24 @@ do {								\
 		kvfree_call_rcu(NULL, (void *) (___p));		\
 } while (0)
 
+/**
+ * kfree_rcu_nolock() - a version of kfree_rcu() that can be called in any context.
+ * @ptr: pointer to kfree for double-argument invocations.
+ * @kvrhf: the name of the struct kvfree_rcu_head within the type of @ptr.
+ *
+ * Objects that are not allocated via kmalloc_nolock() are not supported.
+ * kfree_rcu_nolock() supports 2-arg variant only.
+ */
+#define kfree_rcu_nolock(ptr, kvrhf)						\
+do {										\
+	typeof (ptr) ___p = (ptr);						\
+										\
+	if (___p) {								\
+		BUILD_BUG_ON(offsetof(typeof(*(ptr)), kvrhf) >= 4096);		\
+		kfree_call_rcu_nolock(&((___p)->kvrhf), (void *) (___p));	\
+	}									\
+} while (0)
+
 /*
  * Place this after a lock-acquisition primitive to guarantee that
  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
diff --git a/mm/slab_common.c b/mm/slab_common.c
index c1c32909fa52..09c457645e03 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1263,6 +1263,29 @@ EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
 EXPORT_TRACEPOINT_SYMBOL(kfree);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
 
+void kfree_call_rcu_nolock(struct kvfree_rcu_head *head, void *ptr)
+{
+	struct slab *slab;
+	struct kmem_cache *s;
+
+	VM_WARN_ON_ONCE(is_vmalloc_addr(ptr) || !virt_to_slab(ptr));
+
+	slab = virt_to_slab(ptr);
+	s = slab->slab_cache;
+
+	if (unlikely(IS_ENABLED(CONFIG_NUMA) && slab_nid(slab) != numa_mem_id()))
+		goto fallback;
+
+	if (unlikely(!__kfree_rcu_sheaf(s, ptr, SLAB_FREE_NOLOCK)))
+		goto fallback;
+
+	return;
+
+fallback:
+	defer_kfree_rcu(head);
+}
+EXPORT_SYMBOL_GPL(kfree_call_rcu_nolock);
+
 #ifndef CONFIG_KVFREE_RCU_BATCHED
 
 void kvfree_call_rcu(struct rcu_head *head, void *ptr)
@@ -2120,10 +2143,11 @@ void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
 		cpus_read_lock();
 		flush_rcu_sheaves_on_cache(s);
 		cpus_read_unlock();
-		deferred_work_barrier();
-		rcu_barrier();
 	}
 
+	/* kfree_rcu_nolock() might have deferred frees even without sheaves */
+	deferred_work_barrier();
+	rcu_barrier();
 	__kvfree_rcu_barrier();
 }
 EXPORT_SYMBOL_GPL(kvfree_rcu_barrier_on_cache);
diff --git a/mm/slub.c b/mm/slub.c
index d0976396d703..7114451dd5d0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4081,6 +4081,7 @@ static void flush_all(struct kmem_cache *s)
 
 struct deferred_percpu_work {
 	struct llist_head objects;
+	struct llist_head objects_by_rcu;
 	struct llist_head rcu_sheaves;
 	struct irq_work work;
 };
@@ -4089,6 +4090,7 @@ static void deferred_percpu_work_fn(struct irq_work *work);
 
 static DEFINE_PER_CPU(struct deferred_percpu_work, deferred_percpu_work) = {
 	.objects = LLIST_HEAD_INIT(objects),
+	.objects_by_rcu = LLIST_HEAD_INIT(objects_by_rcu),
 	.rcu_sheaves = LLIST_HEAD_INIT(rcu_sheaves),
 	.work = IRQ_WORK_INIT(deferred_percpu_work_fn),
 };
@@ -6386,13 +6388,14 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
 static void deferred_percpu_work_fn(struct irq_work *work)
 {
 	struct deferred_percpu_work *dpw;
-	struct llist_head *objs, *rcu_sheaves;
+	struct llist_head *objs, *objs_by_rcu, *rcu_sheaves;
 	struct llist_node *llnode, *pos, *t;
 	struct slab_sheaf *sheaf, *next;
 
 	dpw = container_of(work, struct deferred_percpu_work, work);
 	rcu_sheaves = &dpw->rcu_sheaves;
 	objs = &dpw->objects;
+	objs_by_rcu = &dpw->objects_by_rcu;
 
 	llnode = llist_del_all(objs);
 	llist_for_each_safe(pos, t, llnode) {
@@ -6417,6 +6420,14 @@ static void deferred_percpu_work_fn(struct irq_work *work)
 		stat(s, FREE_SLOWPATH);
 	}
 
+	llnode = llist_del_all(objs_by_rcu);
+	llist_for_each_safe(pos, t, llnode) {
+		void *head = pos;
+		void *objp = kvmalloc_obj_start_addr(head);
+
+		kvfree_call_rcu(head, objp);
+	}
+
 	llnode = llist_del_all(rcu_sheaves);
 	llist_for_each_entry_safe(sheaf, next, llnode, llnode)
 		call_rcu(&sheaf->rcu_head, rcu_free_sheaf);
@@ -6435,6 +6446,17 @@ static void defer_free(struct kmem_cache *s, void *head)
 		irq_work_queue(&dpw->work);
 }
 
+void defer_kfree_rcu(struct kvfree_rcu_head *head)
+{
+	struct deferred_percpu_work *dpw;
+
+	guard(preempt)();
+
+	dpw = this_cpu_ptr(&deferred_percpu_work);
+	if (llist_add((struct llist_node *)head, &dpw->objects_by_rcu))
+		irq_work_queue(&dpw->work);
+}
+
 void deferred_work_barrier(void)
 {
 	int cpu;

-- 
2.53.0