On Wed, Jun 2, 2021 at 5:27 PM Tvrtko Ursulin tvrtko.ursulin@linux.intel.com wrote:
On 25/05/2021 17:45, Matthew Brost wrote:
On Tue, May 25, 2021 at 11:32:26AM +0100, Tvrtko Ursulin wrote:
- Context pinning code with it's magical two adds, subtract and cmpxchg is
dodgy as well.
Daniele tried to remove this and it proved quite difficult + created even more races in the backend code. This was prior to the pre-pin and post-unpin code which makes this even more difficult to fix as I believe these functions would need to be removed first. Not saying we can't revisit this someday but I personally really like it - it is a clever way to avoid reentering the pin / unpin code while asynchronous things are happening rather than some complex locking scheme. Lastly, this code has proved incredibly stable as I don't think we've had to fix a single thing in this area since we've been using this code internally.
Pretty much same as above. The code like:
static inline void __intel_context_unpin(struct intel_context *ce) { if (!ce->ops->sched_disable) { __intel_context_do_unpin(ce, 1); } else { while (!atomic_add_unless(&ce->pin_count, -1, 1)) { if (atomic_cmpxchg(&ce->pin_count, 1, 2) == 1) { ce->ops->sched_disable(ce); break; } } } }
That's pretty much impenetrable for me and the only thing I can think of here is **ALARM** must be broken! See what others think..
pin_count is a hand-rolled mutex, except not actually a real one, and it's absolutely hiliarous in it's various incarnations (there's one each on i915_vm, vma, obj and probably a few more).
Not the worst one I've seen by far in the code we've merged already. Minimally this needs a comment here and in the struct next to @pin_count to explain where all this is abused, which would already make it better than most of the in-tree ones.
As part of the ttm conversion we have a plan to sunset the "pin_count as a lock" stuff, depending how bad that goes we might need to split up the task for each struct that has such a pin_count.
-Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch