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

From: Christoph Hellwig
Date: Wed May 27 2020 - 01:25:25 EST


From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

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 <khlebnikov@xxxxxxxxxxxxxx>
[hch: rebased]
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
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