On Mon, Aug 05, 2019 at 05:07:41PM +0100, Chris Wilson wrote:
Quoting Christian König (2019-08-05 16:45:54)
@@ -214,16 +214,16 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) return 0;
retry:
seq = read_seqcount_begin(&resv->seq); rcu_read_lock();
fence_excl = rcu_dereference(resv->fence_excl); 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)) {
if (rcu_dereference(resv->fence_excl) != fence_excl) {If I remember my rules correctly, rcu_dereference is a read-data-depends, which only means that a read through the pointer returned by rcu_dereference() is after the retrieval of that pointer. Nothing orders the retrieval of fence_excl vs shared_count (different pointers), so I think the last line should be:
smp_rmb(); if (rcu_access_pointer(resv->fence_excl) != fence_excl)
Also, barriers must have a comment, the comment must be on both barriers (if you don't have two, you're doing it wrong), and each barrier comment must reference its counterpart. -Daniel