[PATCH] dma-buf: fix and rework dma_buf_poll v4 - dri-devel - freedesktop.org experimental mailing list

30 Jun 2021

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
Signed-off-by: Christian König christian.koenig@amd.com
CC: stable@vger.kernel.org
---
 drivers/dma-buf/dma-buf.c | 132 +++++++++++++-------------------------
 include/linux/dma-buf.h   |   2 +-
 2 files changed, 46 insertions(+), 88 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index eadd1eaa2fb5..192c4d34704b 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 +202,19 @@ 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 __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 {
+	struct dma_buf_poll_cb_t *dcb;
    struct dma_buf *dmabuf;
    struct dma_resv *resv;
    struct dma_resv_list *fobj;
-	struct dma_fence *fence_excl;
+	struct dma_fence *fence;
+	unsigned shared_count;
    __poll_t events;
-	unsigned shared_count, seq;
+	int r, i;
dmabuf = file->private_data;
    if (!dmabuf || !dmabuf->resv)
@@ -225,101 +228,56 @@ 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();
+	dcb = events & EPOLLOUT ? &dmabuf->cb_out : &dmabuf->cb_in;
+
+	/* Only queue a new one if we are not still waiting for the old one */
+	spin_lock_irq(&dmabuf->poll.lock);
+	if (dcb->active)
+		events = 0;
+	else
+		dcb->active = events;
+	spin_unlock_irq(&dmabuf->poll.lock);
+	if (!events)
+		return 0;
+
+	dma_resv_lock(resv, NULL);
-	fobj = rcu_dereference(resv->fence);
-	if (fobj)
+	fobj = dma_resv_get_list(resv);
+	if (fobj && events & EPOLLOUT)
    	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;
-	}
-	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;
-
-		spin_lock_irq(&dmabuf->poll.lock);
-		if (dcb->active) {
-			dcb->active |= pevents;
-			events &= ~pevents;
-		} else
-			dcb->active = pevents;
-		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);
-			}
+	for (i = 0; i < 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) {
+			/* Callback queued */
+			events = 0;
+			goto out;
    	}
+		dma_fence_put(fence);
    }
-	if ((events & EPOLLOUT) && shared_count > 0) {
-		struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_shared;
-		int i;
-
-		/* Only queue a new callback if no event has fired yet */
-		spin_lock_irq(&dmabuf->poll.lock);
-		if (dcb->active)
-			events &= ~EPOLLOUT;
-		else
-			dcb->active = EPOLLOUT;
-		spin_unlock_irq(&dmabuf->poll.lock);
-
-		if (!(events & EPOLLOUT))
+	fence = dma_resv_get_excl(resv);
+	if (fence) {
+		dma_fence_get(fence);
+		r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
+		if (!r) {
+			/* Callback queued */
+			events = 0;
    		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)
-			dma_buf_poll_cb(NULL, &dcb->cb);
+		dma_fence_put(fence);
    }
+	/* No callback queued, wake up any additional waiters. */
+	dma_buf_poll_cb(NULL, &dcb->cb);
+
 out:
-	rcu_read_unlock();
+	dma_resv_unlock(resv);
    return events;
 }
@@ -562,8 +520,8 @@ 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;
 };
/**
-- 
2.25.1


    