md: document lifetime of internal rdev pointer.

[sfrench/cifs-2.6.git] / drivers / md / raid5.h

diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 2e6123825095296555e8a73a52d63872f2fa2bff..3f8da26032accce9fdf492b58fc120a94ac31582 100644 (file)
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -450,6 +450,18 @@ enum {
  * HANDLE gets cleared if stripe_handle leaves nothing locked.
  */
 
+/* Note: disk_info.rdev can be set to NULL asynchronously by raid5_remove_disk.
+ * There are three safe ways to access disk_info.rdev.
+ * 1/ when holding mddev->reconfig_mutex
+ * 2/ when resync/recovery/reshape is known to be happening - i.e. in code that
+ *    is called as part of performing resync/recovery/reshape.
+ * 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 disk_info {
        struct md_rdev  *rdev, *replacement;
        struct page     *extra_page; /* extra page to use in prexor */