[PATCH] dma-buf: fix and rework dma_buf_poll v6
Daniel Vetter
daniel.vetter at ffwll.ch
Tue Jul 20 12:04:27 UTC 2021
On Wed, Jul 14, 2021 at 3:21 PM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
> Just a gentle ping. Or have I missed your reply?
Nah just got overwhelmed with dma-resv discussion and took a break. I
still think some igt (or reviewing what we have) would be good. We
could also just merge Jason's import/export series, since he's typed
the igts for that which us dma_buf poll.
-Daniel
>
> Thanks,
> Christian.
>
> Am 09.07.21 um 14:07 schrieb Christian König:
> > Daniel pointed me towards this function and there are multiple obvious problems
> > in the implementation.
> >
> > First of all the retry loop is not working as intended. In general the retry
> > makes only sense if you grab the reference first and then check the sequence
> > values.
> >
> > Then we should always also wait for the exclusive fence.
> >
> > It's also good practice to keep the reference around when installing callbacks
> > to fences you don't own.
> >
> > And last the whole implementation was unnecessary complex and rather hard to
> > understand which could lead to probably unexpected behavior of the IOCTL.
> >
> > Fix all this by reworking the implementation from scratch. Dropping the
> > whole RCU approach and taking the lock instead.
> >
> > Only mildly tested and needs a thoughtful review of the code.
> >
> > v2: fix the reference counting as well
> > v3: keep the excl fence handling as is for stable
> > v4: back to testing all fences, drop RCU
> > v5: handle in and out separately
> > v6: add missing clear of events
> >
> > Signed-off-by: Christian König <christian.koenig at amd.com>
> > CC: stable at vger.kernel.org
> > ---
> > drivers/dma-buf/dma-buf.c | 156 +++++++++++++++++---------------------
> > include/linux/dma-buf.h | 2 +-
> > 2 files changed, 72 insertions(+), 86 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index eadd1eaa2fb5..39e1ef872829 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -72,7 +72,7 @@ static void dma_buf_release(struct dentry *dentry)
> > * If you hit this BUG() it means someone dropped their ref to the
> > * dma-buf while still having pending operation to the buffer.
> > */
> > - BUG_ON(dmabuf->cb_shared.active || dmabuf->cb_excl.active);
> > + BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
> >
> > dmabuf->ops->release(dmabuf);
> >
> > @@ -202,16 +202,57 @@ static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
> > wake_up_locked_poll(dcb->poll, dcb->active);
> > dcb->active = 0;
> > spin_unlock_irqrestore(&dcb->poll->lock, flags);
> > + dma_fence_put(fence);
> > +}
> > +
> > +static bool dma_buf_poll_shared(struct dma_resv *resv,
> > + struct dma_buf_poll_cb_t *dcb)
> > +{
> > + struct dma_resv_list *fobj = dma_resv_get_list(resv);
> > + struct dma_fence *fence;
> > + int i, r;
> > +
> > + if (!fobj)
> > + return false;
> > +
> > + for (i = 0; i < fobj->shared_count; ++i) {
> > + fence = rcu_dereference_protected(fobj->shared[i],
> > + dma_resv_held(resv));
> > + dma_fence_get(fence);
> > + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
> > + if (!r)
> > + return true;
> > + dma_fence_put(fence);
> > + }
> > +
> > + return false;
> > +}
> > +
> > +static bool dma_buf_poll_excl(struct dma_resv *resv,
> > + struct dma_buf_poll_cb_t *dcb)
> > +{
> > + struct dma_fence *fence = dma_resv_get_excl(resv);
> > + int r;
> > +
> > + if (!fence)
> > + return false;
> > +
> > + dma_fence_get(fence);
> > + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
> > + if (!r)
> > + return true;
> > + dma_fence_put(fence);
> > +
> > + return false;
> > }
> >
> > static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
> > {
> > struct dma_buf *dmabuf;
> > struct dma_resv *resv;
> > - struct dma_resv_list *fobj;
> > - struct dma_fence *fence_excl;
> > + unsigned shared_count;
> > __poll_t events;
> > - unsigned shared_count, seq;
> > + int r, i;
> >
> > dmabuf = file->private_data;
> > if (!dmabuf || !dmabuf->resv)
> > @@ -225,101 +266,46 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
> > if (!events)
> > return 0;
> >
> > -retry:
> > - seq = read_seqcount_begin(&resv->seq);
> > - rcu_read_lock();
> > -
> > - fobj = rcu_dereference(resv->fence);
> > - if (fobj)
> > - shared_count = fobj->shared_count;
> > - else
> > - shared_count = 0;
> > - fence_excl = rcu_dereference(resv->fence_excl);
> > - if (read_seqcount_retry(&resv->seq, seq)) {
> > - rcu_read_unlock();
> > - goto retry;
> > - }
> > + dma_resv_lock(resv, NULL);
> >
> > - if (fence_excl && (!(events & EPOLLOUT) || shared_count == 0)) {
> > - struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_excl;
> > - __poll_t pevents = EPOLLIN;
> > -
> > - if (shared_count == 0)
> > - pevents |= EPOLLOUT;
> > + if (events & EPOLLOUT) {
> > + struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_out;
> >
> > + /* Check that callback isn't busy */
> > spin_lock_irq(&dmabuf->poll.lock);
> > - if (dcb->active) {
> > - dcb->active |= pevents;
> > - events &= ~pevents;
> > - } else
> > - dcb->active = pevents;
> > + if (dcb->active)
> > + events &= ~EPOLLOUT;
> > + else
> > + dcb->active = EPOLLOUT;
> > spin_unlock_irq(&dmabuf->poll.lock);
> >
> > - if (events & pevents) {
> > - if (!dma_fence_get_rcu(fence_excl)) {
> > - /* force a recheck */
> > - events &= ~pevents;
> > - dma_buf_poll_cb(NULL, &dcb->cb);
> > - } else if (!dma_fence_add_callback(fence_excl, &dcb->cb,
> > - dma_buf_poll_cb)) {
> > - events &= ~pevents;
> > - dma_fence_put(fence_excl);
> > - } else {
> > - /*
> > - * No callback queued, wake up any additional
> > - * waiters.
> > - */
> > - dma_fence_put(fence_excl);
> > - dma_buf_poll_cb(NULL, &dcb->cb);
> > - }
> > - }
> > + if (events & EPOLLOUT && !dma_buf_poll_shared(resv, dcb) &&
> > + !dma_buf_poll_excl(resv, dcb))
> > + /* No callback queued, wake up any other waiters */
> > + dma_buf_poll_cb(NULL, &dcb->cb);
> > + else
> > + events &= ~EPOLLOUT;
> > }
> >
> > - if ((events & EPOLLOUT) && shared_count > 0) {
> > - struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_shared;
> > - int i;
> > + if (events & EPOLLIN) {
> > + struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_in;
> >
> > - /* Only queue a new callback if no event has fired yet */
> > + /* Check that callback isn't busy */
> > spin_lock_irq(&dmabuf->poll.lock);
> > if (dcb->active)
> > - events &= ~EPOLLOUT;
> > + events &= ~EPOLLIN;
> > else
> > - dcb->active = EPOLLOUT;
> > + dcb->active = EPOLLIN;
> > spin_unlock_irq(&dmabuf->poll.lock);
> >
> > - if (!(events & EPOLLOUT))
> > - goto out;
> > -
> > - for (i = 0; i < shared_count; ++i) {
> > - struct dma_fence *fence = rcu_dereference(fobj->shared[i]);
> > -
> > - if (!dma_fence_get_rcu(fence)) {
> > - /*
> > - * fence refcount dropped to zero, this means
> > - * that fobj has been freed
> > - *
> > - * call dma_buf_poll_cb and force a recheck!
> > - */
> > - events &= ~EPOLLOUT;
> > - dma_buf_poll_cb(NULL, &dcb->cb);
> > - break;
> > - }
> > - if (!dma_fence_add_callback(fence, &dcb->cb,
> > - dma_buf_poll_cb)) {
> > - dma_fence_put(fence);
> > - events &= ~EPOLLOUT;
> > - break;
> > - }
> > - dma_fence_put(fence);
> > - }
> > -
> > - /* No callback queued, wake up any additional waiters. */
> > - if (i == shared_count)
> > + if (events & EPOLLIN && !dma_buf_poll_excl(resv, dcb))
> > + /* No callback queued, wake up any other waiters */
> > dma_buf_poll_cb(NULL, &dcb->cb);
> > + else
> > + events &= ~EPOLLIN;
> > }
> >
> > -out:
> > - rcu_read_unlock();
> > + dma_resv_unlock(resv);
> > return events;
> > }
> >
> > @@ -562,8 +548,8 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
> > dmabuf->owner = exp_info->owner;
> > spin_lock_init(&dmabuf->name_lock);
> > init_waitqueue_head(&dmabuf->poll);
> > - dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll;
> > - dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0;
> > + dmabuf->cb_in.poll = dmabuf->cb_out.poll = &dmabuf->poll;
> > + dmabuf->cb_in.active = dmabuf->cb_out.active = 0;
> >
> > if (!resv) {
> > resv = (struct dma_resv *)&dmabuf[1];
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > index efdc56b9d95f..7e747ad54c81 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -329,7 +329,7 @@ struct dma_buf {
> > wait_queue_head_t *poll;
> >
> > __poll_t active;
> > - } cb_excl, cb_shared;
> > + } cb_in, cb_out;
> > };
> >
> > /**
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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