avoid startup deadlock in scheduler (#59583)

Author: vtjnash

base/lock.jl

@@ -797,7 +797,7 @@ end
 
 
 # share a lock/condition, since we just need it briefly, so some contention is okay
-const PerThreadLock = ThreadSynchronizer()
+const PerThreadLock = Threads.SpinLock()
 """
     OncePerThread{T}(init::Function)() -> T
 
@@ -901,7 +901,15 @@ OncePerThread(initializer) = OncePerThread{Base.promote_op(initializer), typeof(
                 state = @atomic :monotonic ss[tid]
                 while state == PerStateConcurrent
                     # lost race, wait for notification this is done running elsewhere
-                    wait(PerThreadLock) # wait for initializer to finish without releasing this thread
+                    # without releasing this thread
+                    unlock(PerThreadLock)
+                    while state == PerStateConcurrent
+                        # spin loop until ready
+                        ss = @atomic :acquire once.ss
+                        state = @atomic :monotonic ss[tid]
+                        GC.safepoint()
+                    end
+                    lock(PerThreadLock)
                     ss = @atomic :monotonic once.ss
                     state = @atomic :monotonic ss[tid]
                 end
@@ -915,7 +923,6 @@ OncePerThread(initializer) = OncePerThread{Base.promote_op(initializer), typeof(
                         lock(PerThreadLock)
                         ss = @atomic :monotonic once.ss
                         @atomic :release ss[tid] = PerStateErrored
-                        notify(PerThreadLock)
                         rethrow()
                     end
                     # store result and notify waiters
@@ -924,7 +931,6 @@ OncePerThread(initializer) = OncePerThread{Base.promote_op(initializer), typeof(
                     @atomic :release xs[tid] = result
                     ss = @atomic :monotonic once.ss
                     @atomic :release ss[tid] = PerStateHasrun
-                    notify(PerThreadLock)
                 elseif state == PerStateErrored
                     error("OncePerThread initializer failed previously")
                 elseif state != PerStateHasrun