Re: [PATCH 2/2] loop: use task_work for autoclear operation

On 2021/12/23 23:13, Jens Axboe wrote:

Race is the wrong word, I mean ordering issues. If the fput happens
before you queue your task_work, then it'll be run before that.

There are two fput() related to this patch.
fput(file_of_loop_device) or fput(file_of_backing_file), which one?

loop_schedule_rundown() is called from lo_release() from blkdev_put() from
blkdev_close() from __fput() from task_work_run(), after fput(file_of_loop_device)
called task_work_add(TWA_RESUME) for scheduling __fput().

__loop_clr_fd() is called from loop_rundown_callbackfn() from task_work_run(),
after loop_schedule_rundown() called task_work_add(TWA_RESUME) for scheduling
loop_rundown_callbackfn().

fput(file_of_backing_file) is called from __loop_clr_fd(), and
fput(file_of_backing_file) calls task_work_add(TWA_RESUME) for scheduling
__fput().

And __fput() from task_work_run() calls release callback for file_of_backing_file.

fput(file_of_loop_device) happens before loop_schedule_rundown() calls
task_work_add(TWA_RESUME).

fput(file_of_backing_file) happens after loop_schedule_rundown() called
task_work_add(TWA_RESUME).

The release callback for file_of_backing_file will be completed before
close() syscall which triggered fput(file_of_loop_device) returns to
userspace, for these are chain of add_task_work().

As a total sequence, I think there is no ordering issues.
Or, am I missing limitation of task work usage?