md: document lifetime of internal rdev pointer.

[sfrench/cifs-2.6.git] / drivers / md / raid1.h

diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index c7294e7557e038bf5248e8c8bf9ce84cb6afad58..eb84bc68e2fd4c31cc996bc4d399183cd852b8f2 100644 (file)
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -26,6 +26,18 @@
 #define BARRIER_BUCKETS_NR_BITS                (PAGE_SHIFT - ilog2(sizeof(atomic_t)))
 #define BARRIER_BUCKETS_NR             (1<<BARRIER_BUCKETS_NR_BITS)
 
+/* Note: raid1_info.rdev can be set to NULL asynchronously by raid1_remove_disk.
+ * There are three safe ways to access raid1_info.rdev.
+ * 1/ when holding mddev->reconfig_mutex
+ * 2/ when resync/recovery is known to be happening - i.e. in code that is
+ *    called as part of performing resync/recovery.
+ * 3/ while holding rcu_read_lock(), use rcu_dereference to get the pointer
+ *    and if it is non-NULL, increment rdev->nr_pending before dropping the
+ *    RCU lock.
+ * When .rdev is set to NULL, the nr_pending count checked again and if it has
+ * been incremented, the pointer is put back in .rdev.
+ */
+
 struct raid1_info {
        struct md_rdev  *rdev;
        sector_t        head_position;