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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>
This commit is contained in:
NeilBrown 2018-02-03 09:19:30 +11:00 committed by Shaohua Li
parent 4b6c1060ea
commit f2785b527c
3 changed files with 37 additions and 0 deletions
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12
drivers/md/raid1.h
View File

@ -26,6 +26,18 @@
#define BARRIER_BUCKETS_NR_BITS (PAGE_SHIFT - ilog2(sizeof(atomic_t))) #define BARRIER_BUCKETS_NR_BITS (PAGE_SHIFT - ilog2(sizeof(atomic_t)))
#define BARRIER_BUCKETS_NR (1<<BARRIER_BUCKETS_NR_BITS) #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 raid1_info {
struct md_rdev *rdev; struct md_rdev *rdev;
sector_t head_position; sector_t head_position;

13
drivers/md/raid10.h
View File

@ -2,6 +2,19 @@
#ifndef _RAID10_H #ifndef _RAID10_H
#define _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 raid10_info {
struct md_rdev *rdev, *replacement; struct md_rdev *rdev, *replacement;
sector_t head_position; sector_t head_position;

12
drivers/md/raid5.h
View File

@ -450,6 +450,18 @@ enum {
* HANDLE gets cleared if stripe_handle leaves nothing locked. * 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 disk_info {
struct md_rdev *rdev, *replacement; struct md_rdev *rdev, *replacement;
struct page *extra_page; /* extra page to use in prexor */ struct page *extra_page; /* extra page to use in prexor */