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Commit f2785b52 authored by NeilBrown's avatar NeilBrown Committed by Shaohua Li
Browse files

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: default avatarNeilBrown <neilb@suse.com>
Signed-off-by: default avatarShaohua Li <sh.li@alibaba-inc.com>
parent 4b6c1060

drivers/md/raid1.h

+12 −0
Original line number Diff line number Diff line
@@ -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;

drivers/md/raid10.h

+13 −0
Original line number Diff line number Diff line
@@ -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;

drivers/md/raid5.h

+12 −0
Original line number Diff line number Diff line
@@ -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 */