rcu: introduce kfree_rcu()

Many rcu callbacks functions just call kfree() on the base structure.
These functions are trivial, but their size adds up, and furthermore
when they are used in a kernel module, that module must invoke the
high-latency rcu_barrier() function at module-unload time.

The kfree_rcu() function introduced by this commit addresses this issue.
Rather than encoding a function address in the embedded rcu_head
structure, kfree_rcu() instead encodes the offset of the rcu_head
structure within the base structure.  Because the functions are not
allowed in the low-order 4096 bytes of kernel virtual memory, offsets
up to 4095 bytes can be accommodated.  If the offset is larger than
4095 bytes, a compile-time error will be generated in __kfree_rcu().
If this error is triggered, you can either fall back to use of call_rcu()
or rearrange the structure to position the rcu_head structure into the
first 4096 bytes.

Note that the allowable offset might decrease in the future, for example,
to allow something like kmem_cache_free_rcu().

The new kfree_rcu() function can replace code as follows:

	call_rcu(&p->rcu, simple_kfree_callback);

where "simple_kfree_callback()" might be defined as follows:

	void simple_kfree_callback(struct rcu_head *p)
	{
		struct foo *q = container_of(p, struct foo, rcu);

		kfree(q);
	}

with the following:

	kfree_rcu(&p->rcu, rcu);

Note that the "rcu" is the name of a field in the structure being
freed.  The reason for using this rather than passing in a pointer
to the base structure is that the above approach allows better type
checking.

This commit is based on earlier work by Lai Jiangshan and Manfred Spraul:

Lai's V1 patch: http://lkml.org/lkml/2008/9/18/1
Manfred's patch: http://lkml.org/lkml/2009/1/2/115

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

2 files changed, 2 insertions, 2 deletions

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 0c343b9..4d60fbc 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -167,7 +167,7 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 		prefetch(next);
 		debug_rcu_head_unqueue(list);
 		local_bh_disable();
-		list->func(list);
+		__rcu_reclaim(list);
 		local_bh_enable();
 		list = next;
 		RCU_TRACE(cb_count++);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index b579e4f..2c07adb 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1206,7 +1206,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 		next = list->next;
 		prefetch(next);
 		debug_rcu_head_unqueue(list);
-		list->func(list);
+		__rcu_reclaim(list);
 		list = next;
 		if (++count >= rdp->blimit)
 			break;