block, cfq: move io_cq exit/release to blk-ioc.c

With kmem_cache managed by blk-ioc, io_cq exit/release can be moved to
blk-ioc too.  The odd ->io_cq->exit/release() callbacks are replaced
with elevator_ops->elevator_exit_icq_fn() with unlinking from both ioc
and q, and freeing automatically handled by blk-ioc.  The elevator
operation only need to perform exit operation specific to the elevator
- in cfq's case, exiting the cfqq's.

Also, clearing of io_cq's on q detach is moved to block core and
automatically performed on elevator switch and q release.

Because the q io_cq points to might be freed before RCU callback for
the io_cq runs, blk-ioc code should remember to which cache the io_cq
needs to be freed when the io_cq is released.  New field
io_cq->__rcu_icq_cache is added for this purpose.  As both the new
field and rcu_head are used only after io_cq is released and the
q/ioc_node fields aren't, they are put into unions.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 14 insertions, 6 deletions

diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h
index d15ca6591f96..ac390a34c0e7 100644
--- a/include/linux/iocontext.h
+++ b/include/linux/iocontext.h
@@ -14,14 +14,22 @@ struct io_cq {
         struct request_queue    *q;
         struct io_context       *ioc;
 
-        struct list_head        q_node;
-        struct hlist_node       ioc_node;
+        /*
+         * q_node and ioc_node link io_cq through icq_list of q and ioc
+         * respectively.  Both fields are unused once ioc_exit_icq() is
+         * called and shared with __rcu_icq_cache and __rcu_head which are
+         * used for RCU free of io_cq.
+         */
+        union {
+                struct list_head        q_node;
+                struct kmem_cache       *__rcu_icq_cache;
+        };
+        union {
+                struct hlist_node       ioc_node;
+                struct rcu_head         __rcu_head;
+        };
 
         unsigned long           changed;
-        struct rcu_head         rcu_head;
-
-        void (*exit)(struct io_cq *);
-        void (*release)(struct io_cq *);
 };
 
 /*