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authorNeilBrown <neilb@suse.com>2018-02-03 01:19:30 +0300
committerShaohua Li <sh.li@alibaba-inc.com>2018-02-18 21:22:27 +0300
commitf2785b527cda46314805123ddcbc871655b7c4c4 (patch)
tree797db5c31d88db7c308e660356501d1367c4cab8 /drivers/md
parent4b6c1060eaa6495aa5b0032e8f2d51dd936b1257 (diff)
downloadlinux-f2785b527cda46314805123ddcbc871655b7c4c4.tar.xz
md: document lifetime of internal rdev pointer.
The rdev pointer kept in the local 'config' for each for raid1, raid10, raid4/5/6 has non-obvious lifetime rules. Sometimes RCU is needed, sometimes a lock, something nothing. Add documentation to explain this. Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: Shaohua Li <sh.li@alibaba-inc.com>
Diffstat (limited to 'drivers/md')
-rw-r--r--drivers/md/raid1.h12
-rw-r--r--drivers/md/raid10.h13
-rw-r--r--drivers/md/raid5.h12
3 files changed, 37 insertions, 0 deletions
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index c7294e7557e0..eb84bc68e2fd 100644
--- 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;
diff --git a/drivers/md/raid10.h b/drivers/md/raid10.h
index db2ac22ac1b4..e2e8840de9bf 100644
--- a/drivers/md/raid10.h
+++ b/drivers/md/raid10.h
@@ -2,6 +2,19 @@
#ifndef _RAID10_H
#define _RAID10_H
+/* Note: raid10_info.rdev can be set to NULL asynchronously by
+ * raid10_remove_disk.
+ * There are three safe ways to access raid10_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 raid10_info {
struct md_rdev *rdev, *replacement;
sector_t head_position;
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 2e6123825095..3f8da26032ac 100644
--- 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 */