From c54c7374ff44de5e609506aca7c0deae4703b6d1 Mon Sep 17 00:00:00 2001 From: Lyude Paul Date: Tue, 13 Nov 2018 17:46:14 -0500 Subject: [PATCH] drm/dp_mst: Skip validating ports during destruction, just ref 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 Fixes: 263efde31f97 ("drm/dp/mst: Get validated port ref in drm_dp_update_payload_part1()") Reported-by: Jerry Zuo Cc: Jerry Zuo Cc: Harry Wentland Cc: # v4.6+ Reviewed-by: Dave Airlie Link: https://patchwork.freedesktop.org/patch/msgid/20181113224613.28809-1-lyude@redhat.com Signed-off-by: Sean Paul --- 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 0e0df398222d..250d7160826f 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; } -- 2.20.1