Re: [PATCH] md/raid5: protect lockless reshape_progress accesses

["yu kuai" <yukuai@fygo.io>]

Hi,

在 2026/6/27 18:55, Chen Cheng 写道:
> From: Chen Cheng <chencheng@fnnas.com>
>
> During reshape:
> - reshape_request() advances conf->reshape_progress.
> - reshape_request() itself also reads conf->reshape_progress several times
>    while calculating sector_nr, writepos, and readpos for the current
>    reshape step.
> - use_new_offset() reads conf->reshape_progress locklessly to decide
>    whether the current stripe should use data_offset or new_data_offset.
>
> one possible scenario is:
>
> CPU1                            CPU2
> reshape_request()
> -> conf->reshape_progress += delta
>                                  ops_run_io()
>                                  -> use_new_offset(conf, sh)
>                                  -> progress = conf->reshape_progress
>                                  -> decide old layout or new layout
>                                  -> use data_offset or new_data_offset
>
> reshape_progress is the boundary between old and new layout mapping.
>
> If CPU2 observes an unprotected reshape_progress value, it can choose
> the wrong layout offset for the current stripe.
>
> if reshape_request() reads reshape_progress multiple times without
> a stable snapshot, one iteration of reshape can calculate its range from
> inconsistent progress values.
>
> Fixes: 7a6613810785 ("md/raid5: reshape using largest of old and new chunk size")
>
> The race report:
> ==================================================================
> BUG: KCSAN: data-race in ops_run_io / reshape_request
>
> write to 0xffff89a8d3ef2270 of 8 bytes by task 1299 on cpu 7:
>   reshape_request+0x1292/0x17b0
>   raid5_sync_request+0x815/0xa00
>   md_do_sync.cold+0xf8d/0x1516
>   md_thread+0x15a/0x2d0
>   [....]
>
> read to 0xffff89a8d3ef2270 of 8 bytes by task 1292 on cpu 9:
>   ops_run_io+0xc25/0x1960
>   handle_stripe+0x2273/0x4570
>   handle_active_stripes.isra.0+0x6e0/0xa50
>   raid5d+0x7d5/0xb90
>   [....]
>
> value changed: 0x0000000000173700 -> 0x0000000000173800
> ==================================================================
>
> Signed-off-by: Chen Cheng <chencheng@fnnas.com>
> ---
>   drivers/md/raid5.c | 34 +++++++++++++++++++++-------------
>   1 file changed, 21 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index eaee7f206ab8..b6809f1a3ab4 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -1011,11 +1011,11 @@ static void stripe_add_to_batch_list(struct r5conf *conf,
>   /* Determine if 'data_offset' or 'new_data_offset' should be used
>    * in this stripe_head.
>    */
>   static int use_new_offset(struct r5conf *conf, struct stripe_head *sh)
>   {
> -	sector_t progress = conf->reshape_progress;
> +	sector_t progress = READ_ONCE(conf->reshape_progress);
>   	/* Need a memory barrier to make sure we see the value
>   	 * of conf->generation, or ->data_offset that was set before
>   	 * reshape_progress was updated.
>   	 */
>   	smp_rmb();
> @@ -6340,24 +6340,25 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
>   	sector_t writepos, readpos, safepos;
>   	sector_t stripe_addr;
>   	int reshape_sectors;
>   	struct list_head stripes;
>   	sector_t retn;
> +	sector_t reshape_progress = READ_ONCE(conf->reshape_progress);
>   
>   	if (sector_nr == 0) {
>   		/* If restarting in the middle, skip the initial sectors */
>   		if (mddev->reshape_backwards &&
> -		    conf->reshape_progress < raid5_size(mddev, 0, 0)) {
> +		    reshape_progress < raid5_size(mddev, 0, 0)) {
>   			sector_nr = raid5_size(mddev, 0, 0)
> -				- conf->reshape_progress;
> +				- reshape_progress;
>   		} else if (mddev->reshape_backwards &&
> -			   conf->reshape_progress == MaxSector) {
> +			   reshape_progress == MaxSector) {
>   			/* shouldn't happen, but just in case, finish up.*/
>   			sector_nr = MaxSector;
>   		} else if (!mddev->reshape_backwards &&
> -			   conf->reshape_progress > 0)
> -			sector_nr = conf->reshape_progress;
> +			   reshape_progress > 0)
> +			sector_nr = reshape_progress;
>   		sector_div(sector_nr, new_data_disks);
>   		if (sector_nr) {
>   			mddev->curr_resync_completed = sector_nr;
>   			sysfs_notify_dirent_safe(mddev->sysfs_completed);
>   			*skipped = 1;
> @@ -6377,13 +6378,13 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
>   	 * the data about to be copied would over-write the source of
>   	 * the data at the front of the range.  i.e. one new_stripe
>   	 * along from reshape_progress new_maps to after where
>   	 * reshape_safe old_maps to
>   	 */
> -	writepos = conf->reshape_progress;
> +	writepos = reshape_progress;
>   	sector_div(writepos, new_data_disks);
> -	readpos = conf->reshape_progress;
> +	readpos = reshape_progress;
>   	sector_div(readpos, data_disks);
>   	safepos = conf->reshape_safe;
>   	sector_div(safepos, data_disks);
>   	if (mddev->reshape_backwards) {
>   		if (WARN_ON(writepos < reshape_sectors))
> @@ -6404,11 +6405,11 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
>   
>   	/* Having calculated the 'writepos' possibly use it
>   	 * to set 'stripe_addr' which is where we will write to.
>   	 */
>   	if (mddev->reshape_backwards) {
> -		if (WARN_ON(conf->reshape_progress == 0))
> +		if (WARN_ON(reshape_progress == 0))
>   			return MaxSector;
>   
>   		stripe_addr = writepos;
>   		if (WARN_ON((mddev->dev_sectors &
>   		    ~((sector_t)reshape_sectors - 1)) -
> @@ -6514,14 +6515,21 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
>   			set_bit(STRIPE_HANDLE, &sh->state);
>   		}
>   		list_add(&sh->lru, &stripes);
>   	}
>   	spin_lock_irq(&conf->device_lock);
> -	if (mddev->reshape_backwards)
> -		conf->reshape_progress -= reshape_sectors * new_data_disks;
> -	else
> -		conf->reshape_progress += reshape_sectors * new_data_disks;
> +	if (mddev->reshape_backwards) {
> +		sector_t progress = conf->reshape_progress;
> +
> +		progress -= reshape_sectors * new_data_disks;
> +		WRITE_ONCE(conf->reshape_progress, progress);
> +	} else {
> +		sector_t progress = conf->reshape_progress;
> +
> +		progress += reshape_sectors * new_data_disks;
> +		WRITE_ONCE(conf->reshape_progress, progress);
> +	}
>   	spin_unlock_irq(&conf->device_lock);
>   	/* Ok, those stripe are ready. We can start scheduling
>   	 * reads on the source stripes.
>   	 * The source stripes are determined by mapping the first and last
>   	 * block on the destination stripes.

AI report there are still other places, please check:

   1. get_reshape_loc(), the fast path before taking device_lock:

           if (likely(conf->reshape_progress == MaxSector))
                   return LOC_NO_RESHAPE;

      The comment right below it already mentions the 64bit-on-32bit tearing
      concern, so it makes sense to mark it:

           if (likely(READ_ONCE(conf->reshape_progress) == MaxSector))

   2. raid5_make_request():

           if (likely(conf->reshape_progress == MaxSector)) {

      Same - called on every IO, lockless.

   For consistency the `== MaxSector` check in raid5_remove_disk() could use
   READ_ONCE() too, though it's a reconfig path.


-- 
Thanks,
Kuai