The ext4 module uses rcu_call() thus it should use rcu_barrier()on
module unload.

The kmem cache ext4_pspace_cachep is sometimes free'ed using
call_rcu() callbacks.  Thus, we must wait for completion of call_rcu()
before doing kmem_cache_destroy().

I have difficult determining if no new call_rcu() callbacks can be envoked.
Would the maintainer please verify this?

Signed-off-by: Jesper Dangaard Brouer <redacted>
---

 fs/ext4/mballoc.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 519a0a6..e271a9e 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c

@@ -2902,8 +2902,10 @@ int __init init_ext4_mballoc(void)
 
 void exit_ext4_mballoc(void)
 {
-	/* XXX: synchronize_rcu(); */
+	/* Wait for completion of call_rcu()'s on ext4_pspace_cachep */
+	rcu_barrier();
 	kmem_cache_destroy(ext4_pspace_cachep);
+
 	kmem_cache_destroy(ext4_ac_cachep);
 	kmem_cache_destroy(ext4_free_ext_cachep);
 }