Re: [PATCH v2] md/raid5: read batch_head under stripe_lock in make_stripe_request

Hi,

在 2026/6/19 16:11, Chen Cheng 写道:
> From: Chen Cheng <[email protected]>
>
> KCSAN reports race in raid5_make_request() vs. stripe_add_to_batch_list()
>
> Writer flow (stripe_add_to_batch_list):
> 1. grab `head` stripe;
> 2. lock_two_stripes(head, sh);
> 3. re-check stripe_can_batch() for both head and sh, which requires
>     STRIPE_BATCH_READY set on both;
> 4. write head->batch_head = head and sh->batch_head = head;
> 5. unlock_two_stripes.
>
> STRIPE_BATCH_READY is cleared in two places:
> - clear_batch_ready(), at the entry of handle_stripe();
> - __add_stripe_bio(), for non-batchable bios.
> And, both need to acquire `stripe_lock`.
>
> Under stripe_lock, if STRIPE_BATCH_READY is clear, then:
> - New writers cannot install a batch_head;
> - Existing writers have already finished.
> So .. handle_stripe() readers can ready `batch_head` locklessly.

This does not explain the race clearly, I still have no clue yet.

>
> Fix way:
> Writer side make_stripe_request() under STRIPE_BATCH_READY, so , need
> to be protected by stripe_lock when read something..
>
> v1 -> v2:
> - re-expalin how stripe_lock and batch_head work in commit message , and ,
> - modify comment in raid5.h.
>
> Fixs: f4aec6a097387

Weird fix tag again.

>
>
> KCSAN report:
> ======================================
>    BUG: KCSAN: data-race in raid5_make_request / raid5_make_request
>
>    write to 0xffff8f03062432d8 of 8 bytes by task 210246 on cpu 6:
>      raid5_make_request+0x175e/0x2ab0
>      md_handle_request+0x2c5/0x700
>      md_submit_bio+0x126/0x320
>      [.........]
>      btrfs_sync_file+0x181/0x970
>      vfs_fsync_range+0x71/0x110
>      do_fsync+0x46/0xa0
>      __x64_sys_fsync+0x20/0x30
>
>    read to 0xffff8f03062432d8 of 8 bytes by task 210251 on cpu 0:
>      raid5_make_request+0x7c7/0x2ab0
>      md_handle_request+0x2c5/0x700
>      md_submit_bio+0x126/0x320
>      [.........]
>      btrfs_remap_file_range+0x266/0x980
>      vfs_clone_file_range+0x16d/0x610
>      ioctl_file_clone+0x64/0xd0
>      do_vfs_ioctl+0x87f/0xbc0
>      __x64_sys_ioctl+0xb8/0x130
>
>    value changed: 0x0000000000000000 -> 0xffff8f0307798728

Is this a mismatch report?

>
>
> Signed-off-by: Chen Cheng <[email protected]>
> ---
>   drivers/md/raid5.c | 2 ++
>   drivers/md/raid5.h | 8 +++++++-
>   2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 5521051a9425..efc63740f867 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -6108,14 +6108,16 @@ static enum stripe_result make_stripe_request(struct mddev *mddev,
>   		ctx->do_flush = false;
>   	}
>   
>   	set_bit(STRIPE_HANDLE, &sh->state);
>   	clear_bit(STRIPE_DELAYED, &sh->state);
> +	spin_lock_irq(&sh->stripe_lock);
>   	if ((!sh->batch_head || sh == sh->batch_head) &&
>   	    (bi->bi_opf & REQ_SYNC) &&
>   	    !test_and_set_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
>   		atomic_inc(&conf->preread_active_stripes);
> +	spin_unlock_irq(&sh->stripe_lock);
>   
>   	release_stripe_plug(mddev, sh);
>   	return STRIPE_SUCCESS;
>   
>   out_release:
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index 1c7b710fc9c1..9ff825697ba3 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -221,11 +221,17 @@ struct stripe_head {
>   	enum reconstruct_states reconstruct_state;
>   	spinlock_t		stripe_lock;
>   	int			cpu;
>   	struct r5worker_group	*group;
>   
> -	struct stripe_head	*batch_head; /* protected by stripe lock */
> +	/*
> +	 * Writer protected by stripe_lock.
> +	 * Reader hold stripe_lock when STRIPE_BATCH_READY is set.
> +	 * Without STRIPE_BATCH_READY means no concurrent write,
> +	 * lockless read is ok.
> +	 */
> +	struct stripe_head	*batch_head;
>   	spinlock_t		batch_lock; /* only header's lock is useful */
>   	struct list_head	batch_list; /* protected by head's batch lock*/
>   
>   	union {
>   		struct r5l_io_unit	*log_io;

-- 
Thanks,
Kuai