linux-kernel - Re: [PATCH 1/1] drm/qxl: add mutex_lock/mutex_unlock to ensure the order in which resources are rele

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Date:   Wed, 29 Apr 2020 10:28:37 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Caicai <caizhaopeng@...ontech.com>,
        Dave Airlie <airlied@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        virtualization@...ts.linux-foundation.org,
        dri-devel@...ts.freedesktop.org,
        "linux-kernel@...r.kernel.org\"" <linux-kernel@...r.kernel.org>,
        Zhangyueqian <zhangyueqian@...ontech.com>,
        "Denis V. Lunev" <den@...tuozzo.com>
Subject: Re: [PATCH 1/1] drm/qxl: add mutex_lock/mutex_unlock to ensure the
 order in which resources are rele

  Hi,

> > The only way I see for this to happen is that the guest is preempted
> > between qxl_push_{cursor,command}_ring_release() and
> > qxl_release_fence_buffer_objects() calls.  The host can complete the qxl
> > command then, signal the guest, and the IRQ handler calls
> > qxl_release_free_list() before qxl_release_fence_buffer_objects() runs.
> 
> We think the same: qxl_release was freed by garbage collector before
> original thread had called qxl_release_fence_buffer_objects().

Ok, nice, I think we can consider the issue being analyzed then ;)

> > Looking through the code I think it should be safe to simply swap the
> > qxl_release_fence_buffer_objects() +
> > qxl_push_{cursor,command}_ring_release() calls to close that race
> > window.  Can you try that and see if it fixes the bug for you?
> 
> I'm going to prepare and test such patch but I have one question here:
> qxl_push_*_ring_release can be called with  interruptible=true and fail
> How to correctly handle this case? Is the hunk below correct from your POV?

Oh, right, the error code path will be quite different, checking ...

> --- a/drivers/gpu/drm/qxl/qxl_ioctl.c
> +++ b/drivers/gpu/drm/qxl/qxl_ioctl.c
> @@ -261,12 +261,8 @@ static int qxl_process_single_command(struct qxl_device *qdev,
>                         apply_surf_reloc(qdev, &reloc_info[i]);
>         }
>  
> +       qxl_release_fence_buffer_objects(release);
>         ret = qxl_push_command_ring_release(qdev, release, cmd->type, true);
> -       if (ret)
> -               qxl_release_backoff_reserve_list(release);  <<<< ????
> -       else
> -               qxl_release_fence_buffer_objects(release);
> -
>  out_free_bos:
>  out_free_release:
	if (ret)
		qxl_release_free(qdev, release);

[ code context added ]

qxl_release_free() checks whenever a release is fenced and signals the
fence in case it is so it doesn't wait for the signal forever.  So, yes,
I think qxl_release_free() should cleanup the release properly in any
case and the patch chunk should be correct.

take care,
  Gerd