linux-kernel - Re: [PATCH] drm/atomic: swap_state should stall on cleanup

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Message-ID: <4c71b8b5-fc37-f00d-92cb-551231a5c915@linux.intel.com>
Date:   Fri, 1 Nov 2019 15:56:10 +0100
From:   Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     Rob Clark <robdclark@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        Maxime Ripard <mripard@...nel.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/atomic: swap_state should stall on cleanup_done

Op 31-10-2019 om 23:36 schreef Rob Clark:
> From: Rob Clark <robdclark@...omium.org>
>
> Stalling on cleanup_done ensures that any atomic state related to a
> nonblock commit no longer has dangling references to per-object state
> that can be freed.
>
> Otherwise, if a !nonblock commit completes after a nonblock commit has
> swapped state (ie. the synchronous part of the nonblock commit comes
> before the !nonblock commit), but before the asynchronous part of the
> nonblock commit completes, what was the new per-object state in the
> nonblock commit can be freed.
>
> This shows up with the new self-refresh helper, as _update_avg_times()
> dereferences the original old and new crtc_state.
>
> Fixes: d4da4e33341c ("drm: Measure Self Refresh Entry/Exit times to avoid thrashing")
> Cc: Sean Paul <seanpaul@...omium.org>
> Signed-off-by: Rob Clark <robdclark@...omium.org>
> ---
> Other possibilities:
> 1) maybe block later before freeing atomic state?
> 2) refcount individual per-object state
>
>  drivers/gpu/drm/drm_atomic_helper.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 3ef2ac52ce94..a5d95429f91b 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -2711,7 +2711,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>  			if (!commit)
>  				continue;
>  
> -			ret = wait_for_completion_interruptible(&commit->hw_done);
> +			ret = wait_for_completion_interruptible(&commit->cleanup_done);
>  			if (ret)
>  				return ret;
>  		}
> @@ -2722,7 +2722,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>  			if (!commit)
>  				continue;
>  
> -			ret = wait_for_completion_interruptible(&commit->hw_done);
> +			ret = wait_for_completion_interruptible(&commit->cleanup_done);
>  			if (ret)
>  				return ret;
>  		}
> @@ -2733,7 +2733,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>  			if (!commit)
>  				continue;
>  
> -			ret = wait_for_completion_interruptible(&commit->hw_done);
> +			ret = wait_for_completion_interruptible(&commit->cleanup_done);
>  			if (ret)
>  				return ret;
>  		}

>From setup_commit():

 * Completion of the hardware commit step must be signalled using
 * drm_atomic_helper_commit_hw_done(). After this step the driver is not allowed
 * to read or change any permanent software or hardware modeset state. The only
 * exception is state protected by other means than &drm_modeset_lock locks.
 * Only the free standing @state with pointers to the old state structures can
 * be inspected, e.g. to clean up old buffers using
 * drm_atomic_helper_cleanup_planes().

And the hw_done function says pretty much the same thing. :)