Date:   Thu, 22 Nov 2018 06:08:07 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Lyude Paul <lyude@...hat.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>, sean@...rly.run,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>, Dave Airlie <airlied@...ux.ie>,
        Jerry.Zuo@....com
Subject: Re: [PATCH] drm/dp_mst: Skip validating ports during destruction,
 just ref

On Wed, 14 Nov 2018 at 08:47, Lyude Paul <lyude@...hat.com> wrote:
>
> Jerry Zuo pointed out a rather obscure hotplugging issue that it seems I
> accidentally introduced into DRM two years ago.
>
> Pretend we have a topology like this:
>
> |- DP-1: mst_primary
>    |- DP-4: active display
>    |- DP-5: disconnected
>    |- DP-6: active hub
>       |- DP-7: active display
>       |- DP-8: disconnected
>       |- DP-9: disconnected
>
> If we unplug DP-6, the topology starting at DP-7 will be destroyed but
> it's payloads will live on in DP-1's VCPI allocations and thus require
> removal. However, this removal currently fails because
> drm_dp_update_payload_part1() will (rightly so) try to validate the port
> before accessing it, fail then abort. If we keep going, eventually we
> run the MST hub out of bandwidth and all new allocations will start to
> fail (or in my case; all new displays just start flickering a ton).
>
> We could just teach drm_dp_update_payload_part1() not to drop the port
> ref in this case, but then we also need to teach
> drm_dp_destroy_payload_step1() to do the same thing, then hope no one
> ever adds anything to the that requires a validated port reference in
> drm_dp_destroy_connector_work(). Kind of sketchy.
>
> So let's go with a more clever solution: any port that
> drm_dp_destroy_connector_work() interacts with is guaranteed to still
> exist in memory until we say so. While said port might not be valid we
> don't really care: that's the whole reason we're destroying it in the
> first place! So, teach drm_dp_get_validated_port_ref() to use the all
> mighty current_work() function to avoid attempting to validate ports
> from the context of mgr->destroy_connector_work. I can't see any
> situation where this wouldn't be safe, and this avoids having to play
> whack-a-mole in the future of trying to work around port validation.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Fixes: 263efde31f97 ("drm/dp/mst: Get validated port ref in drm_dp_update_payload_part1()")
> Reported-by: Jerry Zuo <Jerry.Zuo@....com>
> Cc: Jerry Zuo <Jerry.Zuo@....com>
> Cc: Harry Wentland <Harry.Wentland@....com>
> Cc: <stable@...r.kernel.org> # v4.6+

Much as I constantly feel we are missing some better way to do all of
this, this seems like a reasonable fix.

Reviewed-by: Dave Airlie <airlied@...hat.com>

> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 529414556962..08978ad72f33 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1023,9 +1023,20 @@ static struct drm_dp_mst_port *drm_dp_mst_get_port_ref_locked(struct drm_dp_mst_
>  static struct drm_dp_mst_port *drm_dp_get_validated_port_ref(struct drm_dp_mst_topology_mgr *mgr, struct drm_dp_mst_port *port)
>  {
>         struct drm_dp_mst_port *rport = NULL;
> +
>         mutex_lock(&mgr->lock);
> -       if (mgr->mst_primary)
> -               rport = drm_dp_mst_get_port_ref_locked(mgr->mst_primary, port);
> +       /*
> +        * Port may or may not be 'valid' but we don't care about that when
> +        * destroying the port and we are guaranteed that the port pointer
> +        * will be valid until we've finished
> +        */
> +       if (current_work() == &mgr->destroy_connector_work) {
> +               kref_get(&port->kref);
> +               rport = port;
> +       } else if (mgr->mst_primary) {
> +               rport = drm_dp_mst_get_port_ref_locked(mgr->mst_primary,
> +                                                      port);
> +       }
>         mutex_unlock(&mgr->lock);
>         return rport;
>  }
> --
> 2.19.1
>
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