diff mbox series

[1/2] drm/amd/display: Avoid race between dcn10_set_drr() and dc_state_destruct()

Message ID 7b9dbbbb1e6a3aa6d7a4d9367d44d18ddd947158.1725269643.git.tjakobi@math.uni-bielefeld.de (mailing list archive)
State New, archived
Headers show
Series drm/amd: fix VRR race condition during IRQ handling | expand

Commit Message

Tobias Jakobi Sept. 2, 2024, 9:40 a.m. UTC 
From: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>

dc_state_destruct() nulls the resource context of the DC state. The pipe
context passed to dcn10_set_drr() is a member of this resource context.

If dc_state_destruct() is called parallel to the IRQ processing (which
calls dcn10_set_drr() at some point), we can end up using already nulled
function callback fields of struct stream_resource.

The logic in dcn10_set_drr() already tries to avoid this, by checking tg
against NULL. But if the nulling happens exactly after the NULL check and
before the next access, then we get a race.

Avoid this by copying tg first to a local variable, and then use this
variable for all the operations. This should work, as long as nobody
frees the resource pool where the timing generators live.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142
Fixes: 06ad7e164256 ("drm/amd/display: Destroy DC context while keeping DML and DML2")
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
---
 .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

Comments

Sefa Eyeoglu Sept. 2, 2024, 11:03 a.m. UTC | #1 
On Mon, 2024-09-02 at 11:40 +0200, tjakobi@math.uni-bielefeld.de wrote:
> From: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
> 
> dc_state_destruct() nulls the resource context of the DC state. The
> pipe
> context passed to dcn10_set_drr() is a member of this resource
> context.
> 
> If dc_state_destruct() is called parallel to the IRQ processing
> (which
> calls dcn10_set_drr() at some point), we can end up using already
> nulled
> function callback fields of struct stream_resource.
> 
> The logic in dcn10_set_drr() already tries to avoid this, by checking
> tg
> against NULL. But if the nulling happens exactly after the NULL check
> and
> before the next access, then we get a race.
> 
> Avoid this by copying tg first to a local variable, and then use this
> variable for all the operations. This should work, as long as nobody
> frees the resource pool where the timing generators live.
> 
> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142
> Fixes: 06ad7e164256 ("drm/amd/display: Destroy DC context while
> keeping DML and DML2")
> Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
> ---
>  .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++------
> --
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> index 3306684e805a..da8f2cb3c5db 100644
> --- a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> @@ -3223,15 +3223,19 @@ void dcn10_set_drr(struct pipe_ctx
> **pipe_ctx,
>  	 * as well.
>  	 */
>  	for (i = 0; i < num_pipes; i++) {
> -		if ((pipe_ctx[i]->stream_res.tg != NULL) &&
> pipe_ctx[i]->stream_res.tg->funcs) {
> -			if (pipe_ctx[i]->stream_res.tg->funcs-
> >set_drr)
> -				pipe_ctx[i]->stream_res.tg->funcs-
> >set_drr(
> -					pipe_ctx[i]->stream_res.tg,
> &params);
> +		/* dc_state_destruct() might null the stream
> resources, so fetch tg
> +		 * here first to avoid a race condition. The
> lifetime of the pointee
> +		 * itself (the timing_generator object) is not a
> problem here.
> +		 */
> +		struct timing_generator *tg = pipe_ctx[i]-
> >stream_res.tg;
> +
> +		if ((tg != NULL) && tg->funcs) {
> +			if (tg->funcs->set_drr)
> +				tg->funcs->set_drr(tg, &params);
>  			if (adjust.v_total_max != 0 &&
> adjust.v_total_min != 0)
> -				if (pipe_ctx[i]->stream_res.tg-
> >funcs->set_static_screen_control)
> -					pipe_ctx[i]->stream_res.tg-
> >funcs->set_static_screen_control(
> -						pipe_ctx[i]-
> >stream_res.tg,
> -						event_triggers,
> num_frames);
> +				if (tg->funcs-
> >set_static_screen_control)
> +					tg->funcs-
> >set_static_screen_control(
> +						tg, event_triggers,
> num_frames);
>  		}
>  	}
>  }

This fixes the panics with my RX 6800 XT on Sway with VRR enabled!

Tested-by: Sefa Eyeoglu <contact@scrumplex.net>
raoul.van.rueschen@gmail.com Sept. 2, 2024, 2:43 p.m. UTC | #2 
> From: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
>
> dc_state_destruct() nulls the resource context of the DC state. The
> pipe
> context passed to dcn10_set_drr() is a member of this resource
> context.
>
> If dc_state_destruct() is called parallel to the IRQ processing
> (which
> calls dcn10_set_drr() at some point), we can end up using already
> nulled
> function callback fields of struct stream_resource.
>
> The logic in dcn10_set_drr() already tries to avoid this, by checking
> tg
> against NULL. But if the nulling happens exactly after the NULL check
> and
> before the next access, then we get a race.
>
> Avoid this by copying tg first to a local variable, and then use this
> variable for all the operations. This should work, as long as nobody
> frees the resource pool where the timing generators live.
>
> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142
> Fixes: 06ad7e164256 ("drm/amd/display: Destroy DC context while
> keeping DML and DML2")
> Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
> ---
>  .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++------
> --
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> index 3306684e805a..da8f2cb3c5db 100644
> --- a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> @@ -3223,15 +3223,19 @@ void dcn10_set_drr(struct pipe_ctx
> **pipe_ctx,
>  	 * as well.
>  	 */
>  	for (i = 0; i < num_pipes; i++) {
> -		if ((pipe_ctx[i]->stream_res.tg != NULL) &&
> pipe_ctx[i]->stream_res.tg->funcs) {
> -			if (pipe_ctx[i]->stream_res.tg->funcs-
> >set_drr)
> -				pipe_ctx[i]->stream_res.tg->funcs-
> >set_drr(
> -					pipe_ctx[i]->stream_res.tg,
> &params);
> +		/* dc_state_destruct() might null the stream
> resources, so fetch tg
> +		 * here first to avoid a race condition. The
> lifetime of the pointee
> +		 * itself (the timing_generator object) is not a
> problem here.
> +		 */
> +		struct timing_generator *tg = pipe_ctx[i]-
> >stream_res.tg;
> +
> +		if ((tg != NULL) && tg->funcs) {
> +			if (tg->funcs->set_drr)
> +				tg->funcs->set_drr(tg, &params);
>  			if (adjust.v_total_max != 0 &&
> adjust.v_total_min != 0)
> -				if (pipe_ctx[i]->stream_res.tg-
> >funcs->set_static_screen_control)
> -					pipe_ctx[i]->stream_res.tg-
> >funcs->set_static_screen_control(
> -						pipe_ctx[i]-
> >stream_res.tg,
> -						event_triggers,
> num_frames);
> +				if (tg->funcs-
> >set_static_screen_control)
> +					tg->funcs-
> >set_static_screen_control(
> +						tg, event_triggers,
> num_frames);
>  		}
>  	}
>  }

This fixes full system freezes when taking screenshots at low framerates with VRR enabled on an RX 7900 XTX.

Tested-by: Raoul van Rüschen <raoul.van.rueschen@gmail.com>
Christopher Snowhill Sept. 3, 2024, 5:13 a.m. UTC | #3 
On Mon Sep 2, 2024 at 2:40 AM PDT, tjakobi wrote:
> From: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
>
> dc_state_destruct() nulls the resource context of the DC state. The pipe
> context passed to dcn10_set_drr() is a member of this resource context.
>
> If dc_state_destruct() is called parallel to the IRQ processing (which
> calls dcn10_set_drr() at some point), we can end up using already nulled
> function callback fields of struct stream_resource.
>
> The logic in dcn10_set_drr() already tries to avoid this, by checking tg
> against NULL. But if the nulling happens exactly after the NULL check and
> before the next access, then we get a race.
>
> Avoid this by copying tg first to a local variable, and then use this
> variable for all the operations. This should work, as long as nobody
> frees the resource pool where the timing generators live.
>
> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142
> Fixes: 06ad7e164256 ("drm/amd/display: Destroy DC context while keeping DML and DML2")
> Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
> ---
>  .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> index 3306684e805a..da8f2cb3c5db 100644
> --- a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> @@ -3223,15 +3223,19 @@ void dcn10_set_drr(struct pipe_ctx **pipe_ctx,
>  	 * as well.
>  	 */
>  	for (i = 0; i < num_pipes; i++) {
> -		if ((pipe_ctx[i]->stream_res.tg != NULL) && pipe_ctx[i]->stream_res.tg->funcs) {
> -			if (pipe_ctx[i]->stream_res.tg->funcs->set_drr)
> -				pipe_ctx[i]->stream_res.tg->funcs->set_drr(
> -					pipe_ctx[i]->stream_res.tg, &params);
> +		/* dc_state_destruct() might null the stream resources, so fetch tg
> +		 * here first to avoid a race condition. The lifetime of the pointee
> +		 * itself (the timing_generator object) is not a problem here.
> +		 */
> +		struct timing_generator *tg = pipe_ctx[i]->stream_res.tg;
> +
> +		if ((tg != NULL) && tg->funcs) {
> +			if (tg->funcs->set_drr)
> +				tg->funcs->set_drr(tg, &params);
>  			if (adjust.v_total_max != 0 && adjust.v_total_min != 0)
> -				if (pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control)
> -					pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control(
> -						pipe_ctx[i]->stream_res.tg,
> -						event_triggers, num_frames);
> +				if (tg->funcs->set_static_screen_control)
> +					tg->funcs->set_static_screen_control(
> +						tg, event_triggers, num_frames);
>  		}
>  	}
>  }

This fixes hard to trace panics with labwc VRR and Wayfire on RX 6700 XT. I had to use netconsole to arrive at the original bug report.

Tested-by: Christopher Snowhill <chris@kode54.net>
Harry Wentland Sept. 3, 2024, 9:27 p.m. UTC | #4 
On 2024-09-02 05:40, tjakobi@math.uni-bielefeld.de wrote:
> From: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
> 
> dc_state_destruct() nulls the resource context of the DC state. The pipe
> context passed to dcn10_set_drr() is a member of this resource context.
> 
> If dc_state_destruct() is called parallel to the IRQ processing (which
> calls dcn10_set_drr() at some point), we can end up using already nulled
> function callback fields of struct stream_resource.
> 
> The logic in dcn10_set_drr() already tries to avoid this, by checking tg
> against NULL. But if the nulling happens exactly after the NULL check and
> before the next access, then we get a race.
> 
> Avoid this by copying tg first to a local variable, and then use this
> variable for all the operations. This should work, as long as nobody
> frees the resource pool where the timing generators live.
> 
> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142
> Fixes: 06ad7e164256 ("drm/amd/display: Destroy DC context while keeping DML and DML2")
> Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>

Thanks for this fix. It also makes the code more readable.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>

Harry

> ---
>  .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> index 3306684e805a..da8f2cb3c5db 100644
> --- a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> @@ -3223,15 +3223,19 @@ void dcn10_set_drr(struct pipe_ctx **pipe_ctx,
>  	 * as well.
>  	 */
>  	for (i = 0; i < num_pipes; i++) {
> -		if ((pipe_ctx[i]->stream_res.tg != NULL) && pipe_ctx[i]->stream_res.tg->funcs) {
> -			if (pipe_ctx[i]->stream_res.tg->funcs->set_drr)
> -				pipe_ctx[i]->stream_res.tg->funcs->set_drr(
> -					pipe_ctx[i]->stream_res.tg, &params);
> +		/* dc_state_destruct() might null the stream resources, so fetch tg
> +		 * here first to avoid a race condition. The lifetime of the pointee
> +		 * itself (the timing_generator object) is not a problem here.
> +		 */
> +		struct timing_generator *tg = pipe_ctx[i]->stream_res.tg;
> +
> +		if ((tg != NULL) && tg->funcs) {
> +			if (tg->funcs->set_drr)
> +				tg->funcs->set_drr(tg, &params);
>  			if (adjust.v_total_max != 0 && adjust.v_total_min != 0)
> -				if (pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control)
> -					pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control(
> -						pipe_ctx[i]->stream_res.tg,
> -						event_triggers, num_frames);
> +				if (tg->funcs->set_static_screen_control)
> +					tg->funcs->set_static_screen_control(
> +						tg, event_triggers, num_frames);
>  		}
>  	}
>  }
diff mbox series

Patch

diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
index 3306684e805a..da8f2cb3c5db 100644
--- a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
+++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
@@ -3223,15 +3223,19 @@  void dcn10_set_drr(struct pipe_ctx **pipe_ctx,
 	 * as well.
 	 */
 	for (i = 0; i < num_pipes; i++) {
-		if ((pipe_ctx[i]->stream_res.tg != NULL) && pipe_ctx[i]->stream_res.tg->funcs) {
-			if (pipe_ctx[i]->stream_res.tg->funcs->set_drr)
-				pipe_ctx[i]->stream_res.tg->funcs->set_drr(
-					pipe_ctx[i]->stream_res.tg, &params);
+		/* dc_state_destruct() might null the stream resources, so fetch tg
+		 * here first to avoid a race condition. The lifetime of the pointee
+		 * itself (the timing_generator object) is not a problem here.
+		 */
+		struct timing_generator *tg = pipe_ctx[i]->stream_res.tg;
+
+		if ((tg != NULL) && tg->funcs) {
+			if (tg->funcs->set_drr)
+				tg->funcs->set_drr(tg, &params);
 			if (adjust.v_total_max != 0 && adjust.v_total_min != 0)
-				if (pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control)
-					pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control(
-						pipe_ctx[i]->stream_res.tg,
-						event_triggers, num_frames);
+				if (tg->funcs->set_static_screen_control)
+					tg->funcs->set_static_screen_control(
+						tg, event_triggers, num_frames);
 		}
 	}
 }