Re: [PATCH] drm/vkms: Fix race condition around accessing frame number

[Haneen Mohammed <hamohammed.sa@gmail.com>]

On Fri, Aug 31, 2018 at 10:41:40AM +0200, Daniel Vetter wrote:
> On Fri, Aug 24, 2018 at 02:16:34AM +0300, Haneen Mohammed wrote:
> > crtc_state is accessed by both vblank_handle() and the ordered
> > work_struct handle vkms_crc_work_handle() to retrieve and or update
> > the frame number for computed CRC.
> > 
> > Since work_struct can fail, add frame_end to account for missing frame
> > numbers.
> > 
> > use atomic_t witth appropriate flags for synchronization between hrtimer
> > callback and ordered work_struct handle since spinlock can't be used
> > with work_struct handle and mutex can't be used with hrtimer callback.
> > 
> > This patch passes the following subtests from igt kms_pipe_crc_basic test:
> > bad-source, read-crc-pipe-A, read-crc-pipe-A-frame-sequence,
> > nonblocking-crc-pipe-A, nonblocking-crc-pipe-A-frame-sequence
> > 
> > Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
> 
> So atomic_t is probably the greatest trap in the linux kernel. It sounds
> like the right thing, but in 99% of all case you want to use it it isn't.
> The trouble is that atomic_t is _very_ unordered, the only thing it
> guarantees is that atomic_t transactions to the _same_ variable are
> consistent. But anything else can be reordered at will.
> 
> This is very confusing since the new C++ atomic standards has fully
> ordered atomics as the default, and you expressedly need to ask for the
> weakly ordered ones. In linux you need to sprinkle epic amounts of
> smb_barrier* and similar things around them to make atomic_t behave like a
> "normal" C++ atomic type.
> 
> tldr; atomic_t is good for special refcounting needs, when the normal
> refcount_t doesn't cut it. Not much else.
> 
> What usually should be done:
> - Use normal u64 (to match the vblank counter size) instead of atomic_t
>   here.
> - Make sure all access is protect by an appropriate spinlock.
> 

I see, thanks for the explanation!

hm would the patch be fine if I just switch atomic_t to u64 without
using spinlock? since that won't be possible with the work_struct.

Thanks,
Haneen

> > ---
> >  drivers/gpu/drm/vkms/vkms_crc.c  | 33 ++++++++++++++++++++++++++++++--
> >  drivers/gpu/drm/vkms/vkms_crtc.c | 13 +++++++++++--
> >  drivers/gpu/drm/vkms/vkms_drv.h  |  6 ++++--
> >  3 files changed, 46 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c
> > index ed47d67cecd6..4a1ba5b7886a 100644
> > --- a/drivers/gpu/drm/vkms/vkms_crc.c
> > +++ b/drivers/gpu/drm/vkms/vkms_crc.c
> > @@ -34,6 +34,15 @@ static uint32_t _vkms_get_crc(struct vkms_crc_data *crc_data)
> >  	return crc;
> >  }
> >  
> > +/**
> > + * vkms_crc_work_handle - ordered work_struct to compute CRC
> > + *
> > + * @work: work_struct
> > + *
> > + * Work handler for computing CRCs. work_struct scheduled in
> > + * an ordered workqueue that's periodically scheduled to run by
> > + * _vblank_handle() and flushed at vkms_atomic_crtc_destroy_state().
> > + */
> >  void vkms_crc_work_handle(struct work_struct *work)
> >  {
> >  	struct vkms_crtc_state *crtc_state = container_of(work,
> > @@ -45,8 +54,18 @@ void vkms_crc_work_handle(struct work_struct *work)
> >  						output);
> >  	struct vkms_crc_data *primary_crc = NULL;
> >  	struct drm_plane *plane;
> > -
> >  	u32 crc32 = 0;
> > +	u32 frame_start, frame_end;
> > +
> > +	frame_start = atomic_read(&crtc_state->frame_start);
> > +	frame_end = atomic_read(&crtc_state->frame_end);
> > +	/* _vblank_handle() hasn't updated frame_start yet */
> > +	if (!frame_start) {
> 
> I think if we go with u64 we can ignore the issues for wrap-arround, since
> that will simply never happen. But a comment would be good.
> 
> Aside from the atomic_t issue I think this looks good.
> -Daniel
> 
> > +		return;
> > +	} else if (frame_start == frame_end) {
> > +		atomic_set(&crtc_state->frame_start, 0);
> > +		return;
> > +	}
> >  
> >  	drm_for_each_plane(plane, &vdev->drm) {
> >  		struct vkms_plane_state *vplane_state;
> > @@ -67,7 +86,17 @@ void vkms_crc_work_handle(struct work_struct *work)
> >  	if (primary_crc)
> >  		crc32 = _vkms_get_crc(primary_crc);
> >  
> > -	drm_crtc_add_crc_entry(crtc, true, crtc_state->n_frame, &crc32);
> > +	frame_end = drm_crtc_accurate_vblank_count(crtc);
> > +
> > +	/* queue_work can fail to schedule crc_work; add crc for
> > +	 * missing frames
> > +	 */
> > +	while (frame_start <= frame_end)
> > +		drm_crtc_add_crc_entry(crtc, true, frame_start++, &crc32);
> > +
> > +	/* to avoid using the same value again */
> > +	atomic_set(&crtc_state->frame_end, frame_end);
> > +	atomic_set(&crtc_state->frame_start, 0);
> >  }
> >  
> >  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 9d0b1a325a78..a170677acd46 100644
> > --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> > @@ -22,8 +22,17 @@ static void _vblank_handle(struct vkms_output *output)
> >  		DRM_ERROR("vkms failure on handling vblank");
> >  
> >  	if (state && output->crc_enabled) {
> > -		state->n_frame = drm_crtc_accurate_vblank_count(crtc);
> > -		queue_work(output->crc_workq, &state->crc_work);
> > +		u32 frame = drm_crtc_accurate_vblank_count(crtc);
> > +
> > +		/* update frame_start only if a queued vkms_crc_work_handle has
> > +		 * read the data
> > +		 */
> > +		if (!atomic_read(&state->frame_start))
> > +			atomic_set(&state->frame_start, frame);
> > +
> > +		ret = queue_work(output->crc_workq, &state->crc_work);
> > +		if (!ret)
> > +			DRM_WARN("failed to queue vkms_crc_work_handle");
> >  	}
> >  
> >  	spin_unlock(&output->lock);
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> > index 2017a2ccc43d..4a3956a0549e 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.h
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> > @@ -39,12 +39,14 @@ struct vkms_plane_state {
> >   * vkms_crtc_state - Driver specific CRTC state
> >   * @base: base CRTC state
> >   * @crc_work: work struct to compute and add CRC entries
> > - * @n_frame: frame number for computed CRC
> > + * @n_frame_start: start frame number for computed CRC
> > + * @n_frame_end: end frame number for computed CRC
> >   */
> >  struct vkms_crtc_state {
> >  	struct drm_crtc_state base;
> >  	struct work_struct crc_work;
> > -	unsigned int n_frame;
> > +	atomic_t frame_start;
> > +	atomic_t frame_end;
> >  };
> >  
> >  struct vkms_output {
> > -- 
> > 2.17.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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