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md/raid5: protect bitmap batch counters aka seq_flush/seq_write consistency

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kcsan detect race :
- raid5d() closes the current bitmap batch by updating
	conf->seq_flush under conf->device_lock.
- __add_stripe_bio() read conf->seq_flush without that
	lock when assigning sh->bm_seq.

so, protect seq_flush/seq_write consistency for multiple CPUs by
READ_ONCE()/WRITE_ONCE() under the path without held device_lock.

re-explain the stripe batch sequence number update flow:
1. sh->bm_seq declare which batch number the stripe belongs to
   when perform bitmap-related write.
	==> bm_seq = seq_flush+1

2. stripe be handled,
	* if sh->bm_seq - conf->seq_write > 0, means the
	  batch stripes **newer than** the last written
	  batch, it cannot proceed yet, queued on bitmap_list.
	* otherwise , has already proceed.

3. raid5d() `++seq_flush` to closes the current batch, means
	* no more stripes join that old batch
	* just-closed batch ready to write-out to disk

4. raid5d() calls bitmap hooks unplug() or writeout, then,
   `++seq_write` to the same as bm_seq.

- seq_flush - for producer, to close batches.
- seq_write - for consumer, the checkpoint number.

the report:
====================================
BUG: KCSAN: data-race in __add_stripe_bio / raid5d

write to 0xffff88ba5625d470 of 4 bytes by task 82401 on cpu 0:
 raid5d+0x1d9/0xba0
 [.....]

read to 0xffff88ba5625d470 of 4 bytes by task 82421 on cpu 8:
 __add_stripe_bio+0x332/0x400
 raid5_make_request+0x6ac/0x2930
 md_handle_request+0x4a2/0xa40
 md_submit_bio+0x109/0x1a0
 __submit_bio+0x2ec/0x390
 [.....]

Fixes: 7c13edc875 ("md: incorporate new plugging into raid5.")

v1 -> v2:
- remove WRITE_ONCE(conf->seq_write) in held device_lock path.
- remove READ_ONCE(conf->seq_flush) in held device_lock path.

Signed-off-by: Chen Cheng <chencheng@fnnas.com>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260622124649.1780233-1-chencheng@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>
This commit is contained in:
Chen Cheng 2026-06-22 20:46:49 +08:00 committed by Yu Kuai
parent f5098b6bae
commit f565925810
1 changed files with 6 additions and 4 deletions
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10
drivers/md/raid5.c
View File

@ -3553,7 +3553,7 @@ static void __add_stripe_bio(struct stripe_head *sh, struct bio *bi,
sh->dev[dd_idx].sector);
if (conf->mddev->bitmap && firstwrite && !sh->batch_head) {
sh->bm_seq = conf->seq_flush+1;
sh->bm_seq = READ_ONCE(conf->seq_flush) + 1;
set_bit(STRIPE_BIT_DELAY, &sh->state);
}
}
@ -5799,7 +5799,7 @@ static void make_discard_request(struct mddev *mddev, struct bio *bi)
}
spin_unlock_irq(&sh->stripe_lock);
if (conf->mddev->bitmap) {
sh->bm_seq = conf->seq_flush + 1;
sh->bm_seq = READ_ONCE(conf->seq_flush) + 1;
set_bit(STRIPE_BIT_DELAY, &sh->state);
}
@ -6849,12 +6849,14 @@ static void raid5d(struct md_thread *thread)
if (
!list_empty(&conf->bitmap_list)) {
/* Now is a good time to flush some bitmap updates */
conf->seq_flush++;
int seq = conf->seq_flush + 1;
WRITE_ONCE(conf->seq_flush, seq);
spin_unlock_irq(&conf->device_lock);
if (md_bitmap_enabled(mddev, true))
mddev->bitmap_ops->unplug(mddev, true);
spin_lock_irq(&conf->device_lock);
conf->seq_write = conf->seq_flush;
conf->seq_write = seq;
activate_bit_delay(conf, conf->temp_inactive_list);
}
raid5_activate_delayed(conf);