cfq-iosched: remove dead_key from cfq_io_context

Remove ->dead_key field from cfq_io_context to shrink its size to 128 bytes.
(64 bytes for 32-bit hosts)

Use lower bit in ->key as dead-mark, instead of moving key to separate field.
After this for dead cfq_io_context we got cic->key != cfqd automatically.
Thus, io_context's last-hit cache should work without changing.

Now to check ->key for non-dead state compare it with cfqd,
instead of checking ->key for non-null value as it was before.

Plus remove obsolete race protection in cfq_cic_lookup.
This race gone after v2.6.24-1728-g4ac845a

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 28 insertions, 14 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ed897b5ef315..407602350350 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -430,6 +430,23 @@ static inline void cic_set_cfqq(struct cfq_io_context *cic,
         cic->cfqq[is_sync] = cfqq;
 }
 
+#define CIC_DEAD_KEY    1ul
+
+static inline void *cfqd_dead_key(struct cfq_data *cfqd)
+{
+        return (void *)((unsigned long) cfqd | CIC_DEAD_KEY);
+}
+
+static inline struct cfq_data *cic_to_cfqd(struct cfq_io_context *cic)
+{
+        struct cfq_data *cfqd = cic->key;
+
+        if (unlikely((unsigned long) cfqd & CIC_DEAD_KEY))
+                return NULL;
+
+        return cfqd;
+}
+
 /*
  * We regard a request as SYNC, if it's either a read or has the SYNC bit
  * set (in which case it could also be direct WRITE).
@@ -2510,11 +2527,12 @@ static void cfq_cic_free(struct cfq_io_context *cic)
 static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic)
 {
         unsigned long flags;
+        unsigned long dead_key = (unsigned long) cic->key;
 
-        BUG_ON(!cic->dead_key);
+        BUG_ON(!(dead_key & CIC_DEAD_KEY));
 
         spin_lock_irqsave(&ioc->lock, flags);
-        radix_tree_delete(&ioc->radix_root, cic->dead_key);
+        radix_tree_delete(&ioc->radix_root, dead_key & ~CIC_DEAD_KEY);
         hlist_del_rcu(&cic->cic_list);
         spin_unlock_irqrestore(&ioc->lock, flags);
 
@@ -2573,11 +2591,10 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
         list_del_init(&cic->queue_list);
 
         /*
-         * Make sure key == NULL is seen for dead queues
+         * Make sure dead mark is seen for dead queues
          */
         smp_wmb();
-        cic->dead_key = (unsigned long) cic->key;
-        cic->key = NULL;
+        cic->key = cfqd_dead_key(cfqd);
 
         if (ioc->ioc_data == cic)
                 rcu_assign_pointer(ioc->ioc_data, NULL);
@@ -2596,7 +2613,7 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
 static void cfq_exit_single_io_context(struct io_context *ioc,
                                        struct cfq_io_context *cic)
 {
-        struct cfq_data *cfqd = cic->key;
+        struct cfq_data *cfqd = cic_to_cfqd(cic);
 
         if (cfqd) {
                 struct request_queue *q = cfqd->queue;
@@ -2609,7 +2626,7 @@ static void cfq_exit_single_io_context(struct io_context *ioc,
                  * race between exiting task and queue
                  */
                 smp_read_barrier_depends();
-                if (cic->key)
+                if (cic->key == cfqd)
                         __cfq_exit_single_io_context(cfqd, cic);
 
                 spin_unlock_irqrestore(q->queue_lock, flags);
@@ -2689,7 +2706,7 @@ static void cfq_init_prio_data(struct cfq_queue *cfqq, struct io_context *ioc)
 
 static void changed_ioprio(struct io_context *ioc, struct cfq_io_context *cic)
 {
-        struct cfq_data *cfqd = cic->key;
+        struct cfq_data *cfqd = cic_to_cfqd(cic);
         struct cfq_queue *cfqq;
         unsigned long flags;
 
@@ -2746,7 +2763,7 @@ static void cfq_init_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 static void changed_cgroup(struct io_context *ioc, struct cfq_io_context *cic)
 {
         struct cfq_queue *sync_cfqq = cic_to_cfqq(cic, 1);
-        struct cfq_data *cfqd = cic->key;
+        struct cfq_data *cfqd = cic_to_cfqd(cic);
         unsigned long flags;
         struct request_queue *q;
 
@@ -2883,6 +2900,7 @@ cfq_drop_dead_cic(struct cfq_data *cfqd, struct io_context *ioc,
         unsigned long flags;
 
         WARN_ON(!list_empty(&cic->queue_list));
+        BUG_ON(cic->key != cfqd_dead_key(cfqd));
 
         spin_lock_irqsave(&ioc->lock, flags);
 
@@ -2900,7 +2918,6 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
 {
         struct cfq_io_context *cic;
         unsigned long flags;
-        void *k;
 
         if (unlikely(!ioc))
                 return NULL;
@@ -2921,9 +2938,7 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
                 rcu_read_unlock();
                 if (!cic)
                         break;
-                /* ->key must be copied to avoid race with cfq_exit_queue() */
-                k = cic->key;
-                if (unlikely(!k)) {
+                if (unlikely(cic->key != cfqd)) {
                         cfq_drop_dead_cic(cfqd, ioc, cic);
                         rcu_read_lock();
                         continue;
diff --git a/include/linux/iocontext.h b/include/linux/iocontext.h
index a0bb301afac0..64d529133031 100644
--- a/include/linux/iocontext.h
+++ b/include/linux/iocontext.h
@@ -7,7 +7,6 @@
 struct cfq_queue;
 struct cfq_io_context {
         void *key;
-        unsigned long dead_key;
 
         struct cfq_queue *cfqq[2];