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SubjectRe: [PATCH v7 4/4] drm/vc4: Allocate binner bo when starting to use the V3D
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Paul Kocialkowski <paul.kocialkowski@bootlin.com> writes:

> Hi,
>
> On Thu, 2019-04-25 at 10:42 -0700, Eric Anholt wrote:
>> Paul Kocialkowski <paul.kocialkowski@bootlin.com> writes:
>>
>> > The binner BO is not required until the V3D is in use, so avoid
>> > allocating it at probe and do it on the first non-dumb BO allocation.
>> >
>> > Keep track of which clients are using the V3D and liberate the buffer
>> > when there is none left, using a kref. Protect the logic with a
>> > mutex to avoid race conditions.
>> >
>> > The binner BO is created at the time of the first render ioctl and is
>> > destroyed when there is no client and no exec job using it left.
>> >
>> > The Out-Of-Memory (OOM) interrupt also gets some tweaking, to avoid
>> > enabling it before having allocated a binner bo.
>> >
>> > We also want to keep the BO alive during runtime suspend/resume to avoid
>> > failing to allocate it at resume. This happens when the CMA pool is
>> > full at that point and results in a hard crash.
>> >
>> > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
>> > ---
>> > drivers/gpu/drm/vc4/vc4_bo.c | 33 +++++++++++++++++++-
>> > drivers/gpu/drm/vc4/vc4_drv.c | 6 ++++
>> > drivers/gpu/drm/vc4/vc4_drv.h | 14 +++++++++
>> > drivers/gpu/drm/vc4/vc4_gem.c | 13 ++++++++
>> > drivers/gpu/drm/vc4/vc4_irq.c | 21 +++++++++----
>> > drivers/gpu/drm/vc4/vc4_v3d.c | 58 +++++++++++++++++++++++++++--------
>> > 6 files changed, 125 insertions(+), 20 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/vc4/vc4_bo.c b/drivers/gpu/drm/vc4/vc4_bo.c
>> > index 88ebd681d7eb..2b3ec5926fe2 100644
>> > --- a/drivers/gpu/drm/vc4/vc4_bo.c
>> > +++ b/drivers/gpu/drm/vc4/vc4_bo.c
>> > @@ -799,13 +799,38 @@ vc4_prime_import_sg_table(struct drm_device *dev,
>> > return obj;
>> > }
>> >
>> > +static int vc4_grab_bin_bo(struct vc4_dev *vc4, struct vc4_file *vc4file)
>> > +{
>> > + int ret;
>> > +
>> > + if (!vc4->v3d)
>> > + return -ENODEV;
>> > +
>> > + if (vc4file->bin_bo_used)
>> > + return 0;
>> > +
>> > + ret = vc4_v3d_bin_bo_get(vc4);
>> > + if (ret)
>> > + return ret;
>> > +
>> > + vc4file->bin_bo_used = true;
>>
>> I think I found one last race. Multiple threads could be in an ioctl
>> trying to grab the bin BO at the same time (while this is only during
>> app startup, since the fd only needs to get the ref once, it's
>> particularly plausible given that allocating the bin BO is slow). I
>> think if you replace this line with:
>>
>> mutex_lock(&vc4->bin_bo_lock);
>> if (vc4file->bin_bo_used) {
>> mutex_unlock(&vc4->bin_bo_lock);
>> vc4_v3d_bin_bo_put(vc4);
>> } else {
>> vc4file->bin_bo_used = true;
>> mutex_unlock(&vc4->bin_bo_lock);
>> }
>
> Huh, very good catch once again, thanks! It took me some time to grasp
> this one, but as far as I understand, the risk is that we could ref our
> bin bo twice (although it would only be allocated once) since
> bin_bo_used is not protected.
>
> I'd like to suggest another solution, which would avoid re-locking and
> doing an extra put if we got an extra ref: adding a "bool *used"
> argument to vc4_v3d_bin_bo_get and, which only gets dereferenced with
> the bin_bo lock held. Then we can skip obtaining a new reference if
> (used && *used) in vc4_v3d_bin_bo_get.
>
> So we could pass a pointer to vc4file->bin_bo_used for vc4_grab_bin_bo
> and exec->bin_bo_used for the exec case (where there is no such issue
> since we'll only ever try to _get the bin bo once there anyway).
>
> What do you think?

I like it!
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