linux-kernel - Re: [PATCH] ublk: clean up user copy references on ublk server exit

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Message-ID: <CADUfDZqKLvt+uJXSgkwiE3CSew0C5xc6Ft1OfjPdGVQZs3rG-A@mail.gmail.com>
Date: Fri, 19 Dec 2025 11:52:23 -0500
From: Caleb Sander Mateos <csander@...estorage.com>
To: Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>
Cc: Uday Shankar <ushankar@...estorage.com>, linux-block@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ublk: clean up user copy references on ublk server exit

Ming, would you mind taking a (hopefully quick) look at this fix? The
warning reproduces easily with the ublk user copy selftests I recently
added.

Thanks,
Caleb

On Fri, Dec 12, 2025 at 7:19 PM Caleb Sander Mateos
<csander@...estorage.com> wrote:
>
> If a ublk server process releases a ublk char device file, any requests
> dispatched to the ublk server but not yet completed will retain a ref
> value of UBLK_REFCOUNT_INIT. Before commit e63d2228ef83 ("ublk: simplify
> aborting ublk request"), __ublk_fail_req() would decrement the reference
> count before completing the failed request. However, that commit
> optimized __ublk_fail_req() to call __ublk_complete_rq() directly
> without decrementing the request reference count.
> The leaked reference count incorrectly allows user copy and zero copy
> operations on the completed ublk request. It also triggers the
> WARN_ON_ONCE(refcount_read(&io->ref)) warnings in ublk_queue_reinit()
> and ublk_deinit_queue().
> Commit c5c5eb24ed61 ("ublk: avoid ublk_io_release() called after ublk
> char dev is closed") already fixed the issue for ublk devices using
> UBLK_F_SUPPORT_ZERO_COPY or UBLK_F_AUTO_BUF_REG. However, the reference
> count leak also affects UBLK_F_USER_COPY, the other reference-counted
> data copy mode. Fix the condition in ublk_check_and_reset_active_ref()
> to include all reference-counted data copy modes. This ensures that any
> ublk requests still owned by the ublk server when it exits have their
> reference counts reset to 0.
>
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> Fixes: e63d2228ef83 ("ublk: simplify aborting ublk request")
> ---
>  drivers/block/ublk_drv.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index df9831783a13..78f3e22151b9 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -1581,12 +1581,11 @@ static void ublk_set_canceling(struct ublk_device *ub, bool canceling)
>
>  static bool ublk_check_and_reset_active_ref(struct ublk_device *ub)
>  {
>         int i, j;
>
> -       if (!(ub->dev_info.flags & (UBLK_F_SUPPORT_ZERO_COPY |
> -                                       UBLK_F_AUTO_BUF_REG)))
> +       if (!ublk_dev_need_req_ref(ub))
>                 return false;
>
>         for (i = 0; i < ub->dev_info.nr_hw_queues; i++) {
>                 struct ublk_queue *ubq = ublk_get_queue(ub, i);
>
> --
> 2.45.2
>