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Subject[PATCH 5.2 026/137] drm/vkms: Fix crc worker races
Date
From: Daniel Vetter <daniel.vetter@ffwll.ch>

[ Upstream commit 18d0952a838ba559655b0cd9cf85097ad63d9bca ]

The issue we have is that the crc worker might fall behind. We've
tried to handle this by tracking both the earliest frame for which it
still needs to compute a crc, and the last one. Plus when the
crtc_state changes, we have a new work item, which are all run in
order due to the ordered workqueue we allocate for each vkms crtc.

Trouble is there's been a few small issues in the current code:
- we need to capture frame_end in the vblank hrtimer, not in the
worker. The worker might run much later, and then we generate a lot
of crc for which there's already a different worker queued up.
- frame number might be 0, so create a new crc_pending boolean to
track this without confusion.
- we need to atomically grab frame_start/end and clear it, so do that
all in one go. This is not going to create a new race, because if we
race with the hrtimer then our work will be re-run.
- only race that can happen is the following:
1. worker starts
2. hrtimer runs and updates frame_end
3. worker grabs frame_start/end, already reading the new frame_end,
and clears crc_pending
4. hrtimer calls queue_work()
5. worker completes
6. worker gets re-run, crc_pending is false
Explain this case a bit better by rewording the comment.

v2: Demote warning level output to debug when we fail to requeue, this
is expected under high load when the crc worker can't quite keep up.

Cc: Shayenne Moura <shayenneluzmoura@gmail.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Tested-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606222751.32567-2-daniel.vetter@ffwll.ch
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/vkms/vkms_crc.c | 27 +++++++++++++--------------
drivers/gpu/drm/vkms/vkms_crtc.c | 9 +++++++--
drivers/gpu/drm/vkms/vkms_drv.h | 2 ++
3 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c
index d7b409a3c0f8c..66603da634fe3 100644
--- a/drivers/gpu/drm/vkms/vkms_crc.c
+++ b/drivers/gpu/drm/vkms/vkms_crc.c
@@ -166,16 +166,24 @@ void vkms_crc_work_handle(struct work_struct *work)
struct drm_plane *plane;
u32 crc32 = 0;
u64 frame_start, frame_end;
+ bool crc_pending;
unsigned long flags;

spin_lock_irqsave(&out->state_lock, flags);
frame_start = crtc_state->frame_start;
frame_end = crtc_state->frame_end;
+ crc_pending = crtc_state->crc_pending;
+ crtc_state->frame_start = 0;
+ crtc_state->frame_end = 0;
+ crtc_state->crc_pending = false;
spin_unlock_irqrestore(&out->state_lock, flags);

- /* _vblank_handle() hasn't updated frame_start yet */
- if (!frame_start || frame_start == frame_end)
- goto out;
+ /*
+ * We raced with the vblank hrtimer and previous work already computed
+ * the crc, nothing to do.
+ */
+ if (!crc_pending)
+ return;

drm_for_each_plane(plane, &vdev->drm) {
struct vkms_plane_state *vplane_state;
@@ -196,20 +204,11 @@ void vkms_crc_work_handle(struct work_struct *work)
if (primary_crc)
crc32 = _vkms_get_crc(primary_crc, cursor_crc);

- frame_end = drm_crtc_accurate_vblank_count(crtc);
-
- /* queue_work can fail to schedule crc_work; add crc for
- * missing frames
+ /*
+ * The worker can fall behind the vblank hrtimer, make sure we catch up.
*/
while (frame_start <= frame_end)
drm_crtc_add_crc_entry(crtc, true, frame_start++, &crc32);
-
-out:
- /* to avoid using the same value for frame number again */
- spin_lock_irqsave(&out->state_lock, flags);
- crtc_state->frame_end = frame_end;
- crtc_state->frame_start = 0;
- spin_unlock_irqrestore(&out->state_lock, flags);
}

static int vkms_crc_parse_source(const char *src_name, bool *enabled)
diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
index e447b7588d06e..77a1f5fa5d5c8 100644
--- a/drivers/gpu/drm/vkms/vkms_crtc.c
+++ b/drivers/gpu/drm/vkms/vkms_crtc.c
@@ -30,13 +30,18 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
* has read the data
*/
spin_lock(&output->state_lock);
- if (!state->frame_start)
+ if (!state->crc_pending)
state->frame_start = frame;
+ else
+ DRM_DEBUG_DRIVER("crc worker falling behind, frame_start: %llu, frame_end: %llu\n",
+ state->frame_start, frame);
+ state->frame_end = frame;
+ state->crc_pending = true;
spin_unlock(&output->state_lock);

ret = queue_work(output->crc_workq, &state->crc_work);
if (!ret)
- DRM_WARN("failed to queue vkms_crc_work_handle");
+ DRM_DEBUG_DRIVER("vkms_crc_work_handle already queued\n");
}

spin_unlock(&output->lock);
diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
index 81f1cfbeb9362..3c7e06b19efd5 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.h
+++ b/drivers/gpu/drm/vkms/vkms_drv.h
@@ -56,6 +56,8 @@ struct vkms_plane_state {
struct vkms_crtc_state {
struct drm_crtc_state base;
struct work_struct crc_work;
+
+ bool crc_pending;
u64 frame_start;
u64 frame_end;
};
--
2.20.1


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