Re: [PATCH] md: use READ_ONCE() for lockless reads of sb_flags

From: Chen Cheng

Date: Sat Jul 18 2026 - 07:03:08 EST

在 2026/7/12 02:16, yu kuai 写道:
> Hi,
>
> 在 2026/7/11 10:51, Chen Cheng 写道:
>> From: Chen Cheng <chencheng@xxxxxxxxx>
>>
>> sb_flags is checked without a lock in md, raid1, raid5, and raid10.
>> KCSAN reports these reads as data races.
>>
>> The write side uses atomic bit ops.
>> The read side still has plain loads in a few places.
>>
>> Use READ_ONCE() for the lockless reads of sb_flags.
>>
>> KCSAN reports #1:
>> ======================================
>>
>> BUG: KCSAN: data-race in md_check_recovery / md_write_start
>>
>> write (marked) to 0xffff8e39f897f030 of 8 bytes by task 248146 on cpu 8:
>> md_write_start+0x5dd/0x910
>> raid10_make_request+0x9b/0x1080
>> md_handle_request+0x4a2/0xa40
>> [........]
>>
>> read to 0xffff8e39f897f030 of 8 bytes by task 250445 on cpu 11:
>> md_check_recovery+0x574/0x900
>> raid10d+0xb7/0x2950
>> [........]
>>
>> KCSAN reports #2:
>> ======================================
>> BUG: KCSAN: data-race in md_check_recovery / md_write_start
>>
>> write (marked) to 0xffff8e39e953f030 of 8 bytes by task 540091 on cpu 11:
>> md_write_start+0x5dd/0x910
>> raid1_make_request+0x141/0x1990
>> [........]
>>
>> read to 0xffff8e39e953f030 of 8 bytes by task 580822 on cpu 0:
>> md_check_recovery+0x574/0x900
>> raid1d+0xcc/0x3840
>> [........]
>>
>> value changed: 0x0000000000000002 -> 0x0000000000000006
>>
>> KCSAN reports #3:
>> ======================================
>> BUG: KCSAN: data-race in md_check_recovery / md_do_sync.cold
>>
>> write (marked) to 0xffff8e39e9404030 of 8 bytes by task 492473 on cpu 6:
>> md_do_sync.cold+0x3f6/0x1686
>> [........]
>>
>> read to 0xffff8e39e9404030 of 8 bytes by task 492402 on cpu 3:
>> md_check_recovery+0x16d/0x900
>> raid1d+0xcc/0x3840
>> [........]
>>
>> value changed: 0x0000000000000000 -> 0x0000000000000002
>>
>> KCSAN reports #4:
>> ======================================
>> BUG: KCSAN: data-race in md_do_sync.cold / raid5d
>>
>> write (marked) to 0xffff8e39c35cb030 of 8 bytes by task 192196 on cpu 10:
>> md_do_sync.cold+0x3f6/0x1686
>> md_thread+0x15a/0x2d0
>> [........]
>>
>> read to 0xffff8e39c35cb030 of 8 bytes by task 190759 on cpu 5:
>> raid5d+0x7f9/0xba0
>> md_thread+0x15a/0x2d0
>> [........]
>>
>> value changed: 0x0000000000000000 -> 0x0000000000000002
>>
>> Fixes: 2953079c692d ("md: separate flags for superblock changes")
>>
>> Signed-off-by: Chen Cheng <chencheng@xxxxxxxxx>
>> ---
>> v1 -> v2
>> - pass checkpatch.pl check.
>> ---
>> drivers/md/md.c | 8 ++++----
>> drivers/md/raid1.c | 2 +-
>> drivers/md/raid10.c | 4 ++--
>> drivers/md/raid5.c | 7 ++++---
>> 4 files changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index 096bb64e87bd..e72bf2010287 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -6830,11 +6830,11 @@ int md_run(struct mddev *mddev)
>> * via sysfs - until a lack of spares is confirmed.
>> */
>> set_bit(MD_RECOVERY_RECOVER, &mddev->recovery);
>> set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
>>
>> - if (mddev->sb_flags)
>> + if (READ_ONCE(mddev->sb_flags))
>> md_update_sb(mddev, 0);
>>
>> if (IS_ENABLED(CONFIG_MD_BITMAP) && !mddev->bitmap_info.file &&
>> !mddev->bitmap_info.offset)
>> md_bitmap_set_none(mddev);
>> @@ -7024,11 +7024,11 @@ static void __md_stop_writes(struct mddev *mddev)
>> mddev->bitmap_ops->flush(mddev);
>> }
>>
>> if (md_is_rdwr(mddev) &&
>> ((!mddev->in_sync && !mddev_is_clustered(mddev)) ||
>> - mddev->sb_flags)) {
>> + READ_ONCE(mddev->sb_flags))) {
>> /* mark array as shutdown cleanly */
>> if (!mddev_is_clustered(mddev))
>> mddev->in_sync = 1;
>> md_update_sb(mddev, 1);
>> }
>> @@ -10294,11 +10294,11 @@ static bool md_should_do_recovery(struct mddev *mddev)
>> /*
>> * MD_SB_CHANGE_PENDING indicates that the array is switching from clean to
>> * active, and no action is needed for now.
>> * All other MD_SB_* flags require to update the superblock.
>> */
>> - if (mddev->sb_flags & ~ (1<<MD_SB_CHANGE_PENDING))
>> + if (READ_ONCE(mddev->sb_flags) & ~(1<<MD_SB_CHANGE_PENDING))
>> return true;
>
> So this should be a v3, and the above line still not fixed. Please check checkpatch
> warnings.

hi.

I run checkpatch.pl to double-check again .. 0 err 0 warn report .

thanks
cheng.

>
>>
>> /*
>> * If the array is not using external metadata and there has been no data
>> * written for some time, then the array's status needs to be set to
>> @@ -10423,11 +10423,11 @@ void md_check_recovery(struct mddev *mddev)
>> spin_lock(&mddev->lock);
>> set_in_sync(mddev);
>> spin_unlock(&mddev->lock);
>> }
>>
>> - if (mddev->sb_flags)
>> + if (READ_ONCE(mddev->sb_flags))
>> md_update_sb(mddev, 0);
>>
>> /*
>> * Never start a new sync thread if MD_RECOVERY_RUNNING is
>> * still set.
>> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
>> index 29b58583e381..bd6808656edb 100644
>> --- a/drivers/md/raid1.c
>> +++ b/drivers/md/raid1.c
>> @@ -2738,11 +2738,11 @@ static void raid1d(struct md_thread *thread)
>> handle_read_error(conf, r1_bio);
>> else
>> WARN_ON_ONCE(1);
>>
>> cond_resched();
>> - if (mddev->sb_flags & ~(1<<MD_SB_CHANGE_PENDING))
>> + if (READ_ONCE(mddev->sb_flags) & ~(1 << MD_SB_CHANGE_PENDING))
>> md_check_recovery(mddev);
>> }
>> blk_finish_plug(&plug);
>> }
>>
>> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>> index adaf9e432e25..3ffa5a19964d 100644
>> --- a/drivers/md/raid10.c
>> +++ b/drivers/md/raid10.c
>> @@ -3030,11 +3030,11 @@ static void raid10d(struct md_thread *thread)
>> handle_read_error(mddev, r10_bio);
>> else
>> WARN_ON_ONCE(1);
>>
>> cond_resched();
>> - if (mddev->sb_flags & ~(1<<MD_SB_CHANGE_PENDING))
>> + if (READ_ONCE(mddev->sb_flags) & ~(1 << MD_SB_CHANGE_PENDING))
>> md_check_recovery(mddev);
>> }
>> blk_finish_plug(&plug);
>> }
>>
>> @@ -4698,11 +4698,11 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr,
>> else
>> mddev->curr_resync_completed = conf->reshape_progress;
>> conf->reshape_checkpoint = jiffies;
>> set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
>> md_wakeup_thread(mddev->thread);
>> - wait_event(mddev->sb_wait, mddev->sb_flags == 0 ||
>> + wait_event(mddev->sb_wait, READ_ONCE(mddev->sb_flags) == 0 ||
>> test_bit(MD_RECOVERY_INTR, &mddev->recovery));
>> if (test_bit(MD_RECOVERY_INTR, &mddev->recovery)) {
>> allow_barrier(conf);
>> return sectors_done;
>> }
>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>> index ded6a69f7795..cb58b4353995 100644
>> --- a/drivers/md/raid5.c
>> +++ b/drivers/md/raid5.c
>> @@ -1219,11 +1219,11 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
>> break;
>>
>> if (bad < 0) {
>> set_bit(BlockedBadBlocks, &rdev->flags);
>> if (!conf->mddev->external &&
>> - conf->mddev->sb_flags) {
>> + READ_ONCE(conf->mddev->sb_flags)) {
>> /* It is very unlikely, but we might
>> * still need to write out the
>> * bad block log - better give it
>> * a chance*/
>> md_check_recovery(conf->mddev);
>> @@ -6469,11 +6469,11 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
>> rdev->recovery_offset = sector_nr;
>>
>> conf->reshape_checkpoint = jiffies;
>> set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
>> md_wakeup_thread(mddev->thread);
>> - wait_event(mddev->sb_wait, mddev->sb_flags == 0 ||
>> + wait_event(mddev->sb_wait, READ_ONCE(mddev->sb_flags) == 0 ||
>> test_bit(MD_RECOVERY_INTR, &mddev->recovery));
>> if (test_bit(MD_RECOVERY_INTR, &mddev->recovery))
>> return 0;
>> spin_lock_irq(&conf->device_lock);
>> conf->reshape_safe = mddev->reshape_position;
>> @@ -6913,11 +6913,12 @@ static void raid5d(struct md_thread *thread)
>> conf->temp_inactive_list);
>> if (!batch_size && !released)
>> break;
>> handled += batch_size;
>>
>> - if (mddev->sb_flags & ~(1 << MD_SB_CHANGE_PENDING)) {
>> + if (READ_ONCE(mddev->sb_flags) &
>> + ~(1 << MD_SB_CHANGE_PENDING)) {
>> spin_unlock_irq(&conf->device_lock);
>> md_check_recovery(mddev);
>> spin_lock_irq(&conf->device_lock);
>> }
>> }
>