Re: [PATCH RFC] io_uring: improve current file position IO

[Jann Horn <jannh@google.com>]

On Fri, Dec 24, 2021 at 3:35 PM Jens Axboe <axboe@kernel.dk> wrote:
> io_uring should be protecting the current file position with the
> file position lock, ->f_pos_lock. Grab and track the lock state when
> the request is being issued, and make the unlock part of request
> cleaning.
>
> Fixes: ba04291eb66e ("io_uring: allow use of offset == -1 to mean file position")
> Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>
> ---
>
> Main thing I don't like here:
>
> - We're holding the f_pos_lock across the kernel/user boundary, as
>   it's held for the duration of the IO. Alternatively we could
>   keep it local to io_read() and io_write() and lose REQ_F_CUR_POS_LOCK,
>   but will messy up those functions more and add more items to the
>   fast path (which current position read/write definitely is not).
>
> Suggestions welcome...

Oh, that's not pretty... is it guaranteed that the
__f_unlock_pos(req->file) will happen in the same task as the
io_file_pos_lock(req, false), and have you tried running this with
lockdep and mutex debugging enabled? Could a task deadlock if it tried
to do a read() on a file while io_uring is already holding the
position lock?