[PATCH 14/16] block: remove rcu_read_lock() from part_stat_lock()

[Christoph Hellwig]

From: Konstantin Khlebnikov <khlebni...@yandex-team.ru>

The RCU lock is required only in disk_map_sector_rcu() to lookup the
partition.  After that request holds reference to related hd_struct.

Replace get_cpu() with preempt_disable() - returned cpu index is unused.

Signed-off-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
[hch: rebased]
Signed-off-by: Christoph Hellwig <h...@lst.de>
---
 block/genhd.c             | 11 ++++++++---
 include/linux/part_stat.h |  4 ++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 3e7df0a3e6bb0..1a76593276644 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -321,11 +321,12 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk 
*disk, sector_t sector)
        struct hd_struct *part;
        int i;
 
+       rcu_read_lock();
        ptbl = rcu_dereference(disk->part_tbl);
 
        part = rcu_dereference(ptbl->last_lookup);
        if (part && sector_in_part(part, sector) && hd_struct_try_get(part))
-               return part;
+               goto out_unlock;
 
        for (i = 1; i < ptbl->len; i++) {
                part = rcu_dereference(ptbl->part[i]);
@@ -339,10 +340,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk 
*disk, sector_t sector)
                        if (!hd_struct_try_get(part))
                                break;
                        rcu_assign_pointer(ptbl->last_lookup, part);
-                       return part;
+                       goto out_unlock;
                }
        }
-       return &disk->part0;
+
+       part = &disk->part0;
+out_unlock:
+       rcu_read_unlock();
+       return part;
 }
 
 /**
diff --git a/include/linux/part_stat.h b/include/linux/part_stat.h
index 6644197980b92..a6b0938ce82e9 100644
--- a/include/linux/part_stat.h
+++ b/include/linux/part_stat.h
@@ -21,8 +21,8 @@ struct disk_stats {
  *
  * part_stat_read() can be called at any time.
  */
-#define part_stat_lock()       ({ rcu_read_lock(); get_cpu(); })
-#define part_stat_unlock()     do { put_cpu(); rcu_read_unlock(); } while (0)
+#define part_stat_lock()       preempt_disable()
+#define part_stat_unlock()     preempt_enable()
 
 #define part_stat_get_cpu(part, field, cpu)                            \
        (per_cpu_ptr((part)->dkstats, (cpu))->field)
-- 
2.26.2