Re: [PATCH v2] md/raid5: protect bitmap batch counters aka seq_flush/seq_write consistency

[yu kuai]

在 2026/6/17 22:28, Chen Cheng 写道:

> From: Chen Cheng<chencheng@xxxxxxxxx>
>
> 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
>   [.....]
>
> 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@xxxxxxxxx>
> ---
>   drivers/md/raid5.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)

Reviewed-by: Yu Kuai <yukuai@xxxxxxx>

-- 
Thanks,
Kuai