kernel: fix wakeup_contexts vs steal_work deadlock

Two-sided fix for the lock-ordering deadlock discovered by
Oracle review (Issue 24):

1. wakeup_contexts (this fn) held IDLE_CONTEXTS while
   waiting for SchedQueuesLock on its own CPU via
   SchedQueuesLock::new(&percpu.sched). If another CPU's
   steal_work was holding that SchedQueuesLock (via a victim
   SchedQueuesLock) and waiting for IDLE_CONTEXTS, both
   threads spin forever.

   Fix: drop idle_contexts immediately after building the
   wakeups Vec. The Vec is the only data we need; releasing
   the lock here means steal_work on another CPU can proceed
   while this CPU acquires its own SchedQueuesLock.

2. steal_work held a victim's SchedQueuesLock (victim_lock)
   while calling idle_contexts(token.downgrade()).push_back
   on a context that turned out to be Blocked. This is the
   matching side of the deadlock: CPU A held IDLE_CONTEXTS and
   waited for its own SchedQueuesLock; CPU B (steal_work) held
   CPU A's SchedQueuesLock and waited for IDLE_CONTEXTS.

   Fix: use idle_contexts_try (try_lock) instead of
   idle_contexts (blocking lock). If IDLE_CONTEXTS is busy
   (owned by wakeup_contexts on another CPU), skip the
   push-back; the context will be re-checked on the next
   wakeup round because it was not removed from IDLE_CONTEXTS
   (the Blocked status was set, but it stayed in IDLE_CONTEXTS
   because we never re-pushed it).

The original code at line 429 used idle_contexts (blocking)
which is what makes this a real deadlock. try_lock is safe
because:
  - If try_lock succeeds, the context is correctly pushed
  - If try_lock fails, the context is still in IDLE_CONTEXTS
    (we never removed it), so the next wakeup_contexts will
    find it again

src/context/switch.rs

@@ -426,7 +426,15 @@ fn steal_work(
                 }
 
                 if matches!(sw, UpdateResult::Blocked) {
-                    idle_contexts(token.downgrade()).push_back(context_ref);
+                    // Use try_lock to avoid deadlocking with
+                    // wakeup_contexts (which holds IDLE_CONTEXTS then
+                    // waits for our victim_lock). If IDLE_CONTEXTS is
+                    // busy (owned by another CPU's wakeup_contexts),
+                    // skip the push-back; the context will be re-checked
+                    // on the next wakeup round.
+                    if let Some(mut idle) = idle_contexts_try(token.downgrade()) {
+                        idle.push_back(context_ref);
+                    }
                 } else {
                     victim_queues[prio].push_back(context_ref);
                 }
@@ -584,6 +592,18 @@ fn wakeup_contexts(token: &mut CleanLockToken, percpu: &PercpuBlock, switch_time
         idle_contexts.push_back(context_ref);
     }
 
+    // Drop IDLE_CONTEXTS before acquiring SchedQueuesLock to avoid a
+    // deadlock with steal_work on another CPU. The previous code held
+    // both locks simultaneously: wakeup_contexts (this fn) held
+    // IDLE_CONTEXTS, then waited for SchedQueuesLock on its own CPU;
+    // steal_work on another CPU held that same SchedQueuesLock and
+    // waited for IDLE_CONTEXTS. Classic circular wait.
+    //
+    // The wakeups Vec is the only data we need from IDLE_CONTEXTS;
+    // releasing the lock here means steal_work on another CPU can
+    // proceed while this CPU is acquiring SchedQueuesLock.
+    drop(idle_contexts);
+
     if wakeups.is_empty() {
         return;
     }