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
