On Wed, Jul 16, 2025 at 7:48 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> The lockdep tool can report a circular lock dependency warning in the loop
> driver's AIO read/write path:
>
> ```
> [ 6540.587728] kworker/u96:5/72779 is trying to acquire lock:
> [ 6540.593856] ff110001b5968440 (sb_writers#9){.+.+}-{0:0}, at: loop_process_work+0x11a/0xf70 [loop]
> [ 6540.603786]
> [ 6540.603786] but task is already holding lock:
> [ 6540.610291] ff110001b5968440 (sb_writers#9){.+.+}-{0:0}, at: loop_process_work+0x11a/0xf70 [loop]
> [ 6540.620210]
> [ 6540.620210] other info that might help us debug this:
> [ 6540.627499] Possible unsafe locking scenario:
> [ 6540.627499]
> [ 6540.634110] CPU0
> [ 6540.636841] ----
> [ 6540.639574] lock(sb_writers#9);
> [ 6540.643281] lock(sb_writers#9);
> [ 6540.646988]
> [ 6540.646988] *** DEADLOCK ***
> ```
>
> This patch fixes the issue by using the AIO-specific helpers
> `kiocb_start_write()` and `kiocb_end_write()`. These functions are
> designed to be used with a `kiocb` and manage write sequencing
> correctly for asynchronous I/O without introducing the problematic
> lock dependency.
>
> The `kiocb` is already part of the `loop_cmd` struct, so this change
> also simplifies the completion function `lo_rw_aio_do_completion()` by
> using the `iocb` from the `cmd` struct directly, instead of retrieving
> the loop device from the request queue.
>
> Fixes: 39d86db34e41 ("loop: add file_start_write() and file_end_write()")
> Cc: Changhui Zhong <czhong@xxxxxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> drivers/block/loop.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 500840e4a74e..8d994cae3b83 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -308,14 +308,13 @@ static void lo_complete_rq(struct request *rq)
> static void lo_rw_aio_do_completion(struct loop_cmd *cmd)
> {
> struct request *rq = blk_mq_rq_from_pdu(cmd);
> - struct loop_device *lo = rq->q->queuedata;
>
> if (!atomic_dec_and_test(&cmd->ref))
> return;
> kfree(cmd->bvec);
> cmd->bvec = NULL;
> if (req_op(rq) == REQ_OP_WRITE)
> - file_end_write(lo->lo_backing_file);
> + kiocb_end_write(&cmd->iocb);
> if (likely(!blk_should_fake_timeout(rq->q)))
> blk_mq_complete_request(rq);
> }
> @@ -391,7 +390,7 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
> }
>
> if (rw == ITER_SOURCE) {
> - file_start_write(lo->lo_backing_file);
> + kiocb_start_write(&cmd->iocb);
> ret = file->f_op->write_iter(&cmd->iocb, &iter);
> } else
> ret = file->f_op->read_iter(&cmd->iocb, &iter);
> --
> 2.50.1
>
Thanks Ming！
Verify that after applying the patch, this warning message is no
longer triggered，
