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Subject[RFC PATCH] rcu: introduce kfree_rcu()

sometimes a rcu callback is just calling kfree() to free a struct's memory
(we say this callback is a trivial callback.).
this patch introduce kfree_rcu() to do these things directly, easily.

There are 4 reasons that we need kfree_rcu():

1) unloadable modules:
a module(rcu callback is defined in this module) using rcu must
call rcu_barrier() when unload. rcu_barrier() will increase
the system's overhead(the more cpus the worse) and
rcu_barrier() is very time-consuming. if all rcu callback defined
in this module are trivial callback, we can just call kfree_rcu()
instead, save a rcu_barrier() when unload.

2) duplicate code:
all trivial callback are duplicate code though the structs to be freed
are different. it's just a container_of() and a kfree().
There are about 50% callbacks are trivial callbacks for call_rcu() in
current kernel code.

3) cache:
the instructions of trivial callback is not in the cache supposedly.
calling a trivial callback will let to cache missing very likely.
the more trivial callback the more cache missing. OK, this is
not a problem now or in a few days: Only less than 1% trivial callback
are called in running kernel.

4) future:
the number of user of rcu is increasing. new code for rcu is
trivial callback very likely. it means more modules using rcu
and more duplicate code(may come to 90% of callbacks is trivial
callbacks) and more cache missing.

there were a lot of ideas came out when i implemented kfree_rcu().
I chose the simplest one as this patch shows. but these implementation
may cannot be used for to free a struct larger than 16KBytes.

kfree_rcu_bh()? kfree_rcu_sched()?
these two are not need current. call_rcu_bh() & call_rcu_sched()
are hardly be called(and hardly be called for trivial callback).

No, vfree() is not atomic function, will not be called in softirq.

Signed-off-by: Lai Jiangshan <>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index e8b4039..04c654f 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -253,4 +253,25 @@ extern void rcu_barrier_sched(void);
extern void rcu_init(void);
extern int rcu_needs_cpu(int cpu);

+#define __KFREE_RCU_MAX_OFFSET 4095
+#define KFREE_RCU_MAX_OFFSET (sizeof(void *) * __KFREE_RCU_MAX_OFFSET)
+#define __rcu_reclaim(head) \
+do { \
+ unsigned long __offset = (unsigned long)head->func; \
+ if (__offset <= __KFREE_RCU_MAX_OFFSET) \
+ kfree((void *)head - sizeof(void *) * __offset); \
+ else \
+ head->func(head); \
+} while(0)
+ * kfree_rcu - free previously allocated memory after a grace period.
+ * @ptr: pointer returned by kmalloc.
+ * @head: structure to be used for queueing the RCU updates. This structure
+ * is a part of previously allocated memory @ptr.
+ */
+extern void kfree_rcu(const void *ptr, struct rcu_head *head);
#endif /* __LINUX_RCUPDATE_H */
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
index aad93cd..5a14190 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -232,7 +232,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
while (list) {
next = list->next;
- list->func(list);
+ __rcu_reclaim(list);
list = next;
if (++count >= rdp->blimit)
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 467d594..aa9b56a 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -162,6 +162,18 @@ void rcu_barrier_sched(void)

+void kfree_rcu(const void *ptr, struct rcu_head *head)
+ unsigned long offset;
+ typedef void (*rcu_callback)(struct rcu_head *);
+ offset = (void *)head - (void *)ptr;
+ call_rcu(head, (rcu_callback)(offset / sizeof(void *)));
void __init rcu_init(void)
diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
index 2782793..62a9e54 100644
--- a/kernel/rcupreempt.c
+++ b/kernel/rcupreempt.c
@@ -1108,7 +1108,7 @@ static void rcu_process_callbacks(struct softirq_action *unused)
spin_unlock_irqrestore(&rdp->lock, flags);
while (list) {
next = list->next;
- list->func(list);
+ __rcu_reclaim(list);
list = next;

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