rusticl/event: use Weak refs for dependencies

This fixes a potential stack overflow when the dep chain of events gets
too long and droped all at the same time.

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29089>
(cherry picked from commit a45f199086)

.pick_status.json

@@ -894,7 +894,7 @@
         "description": "rusticl/event: use Weak refs for dependencies",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/gallium/frontends/rusticl/core/event.rs

@@ -15,6 +15,7 @@ use std::sync::Arc;
 use std::sync::Condvar;
 use std::sync::Mutex;
 use std::sync::MutexGuard;
+use std::sync::Weak;
 use std::time::Duration;
 
 // we assert that those are a continous range of numbers so we won't have to use HashMaps
@@ -48,7 +49,8 @@ pub struct Event {
     pub context: Arc<Context>,
     pub queue: Option<Arc<Queue>>,
     pub cmd_type: cl_command_type,
-    pub deps: Vec<Arc<Event>>,
+    // using a Weak ref so we don't cause stack overflows in the `drop` impl
+    pub deps: Vec<Weak<Event>>,
     state: Mutex<EventMutState>,
     cv: Condvar,
 }
@@ -66,6 +68,7 @@ impl Event {
         deps: Vec<Arc<Event>>,
         work: EventSig,
     ) -> Arc<Event> {
+        let deps = deps.iter().map(Arc::downgrade).collect();
         Arc::new(Self {
             base: CLObjectBase::new(),
             context: queue.context.clone(),
@@ -238,14 +241,18 @@ impl Event {
         }
     }
 
-    fn deep_unflushed_deps_impl<'a>(&'a self, result: &mut HashSet<&'a Event>) {
+    pub fn deps(&self) -> impl Iterator<Item = Arc<Self>> + Clone + '_ {
+        self.deps.iter().filter_map(Weak::upgrade)
+    }
+
+    fn deep_unflushed_deps_impl(self: &Arc<Self>, result: &mut HashSet<Arc<Event>>) {
         if self.status() <= CL_SUBMITTED as i32 {
             return;
         }
 
         // only scan dependencies if it's a new one
-        if result.insert(self) {
-            for e in &self.deps {
+        if result.insert(Arc::clone(self)) {
+            for e in self.deps() {
                 e.deep_unflushed_deps_impl(result);
             }
         }
@@ -253,7 +260,7 @@ impl Event {
 
     /// does a deep search and returns a list of all dependencies including `events` which haven't
     /// been flushed out yet
-    pub fn deep_unflushed_deps(events: &[Arc<Event>]) -> HashSet<&Event> {
+    pub fn deep_unflushed_deps(events: &[Arc<Event>]) -> HashSet<Arc<Event>> {
         let mut result = HashSet::new();
 
         for e in events {

src/gallium/frontends/rusticl/core/queue.rs

@@ -127,16 +127,17 @@ impl Queue {
                     let mut flushed = Vec::new();
 
                     for e in new_events {
+                        let deps_iter = e.deps();
+
                         // If we hit any deps from another queue, flush so we don't risk a dead
-                        // lock.
-                        if e.deps.iter().any(|ev| ev.queue != e.queue) {
+                        // lock. Also clone the iter here as we'll iterate again later
+                        if deps_iter.clone().any(|ev| ev.queue != e.queue) {
+                            // this flush _has_ to happen before we wait on any of the deps
                             flush_events(&mut flushed, &ctx);
                         }
 
                         // We have to wait on user events or events from other queues.
-                        let err = e
-                            .deps
-                            .iter()
+                        let err = deps_iter
                             .filter(|ev| ev.is_user() || ev.queue != e.queue)
                             .map(|e| e.wait())
                             .find(|s| *s < 0);