Merge pull request #455 from JuliaParallel/jps/errormonitor-spin-lock

errormonitor_tracked: Use spin lock

Author: jpsamaroo

Diff for: src/sch/Sch.jl

@@ -11,7 +11,7 @@ import Base: @invokelatest
 import ..Dagger
 import ..Dagger: Context, Processor, Thunk, WeakThunk, ThunkFuture, ThunkFailedException, Chunk, WeakChunk, OSProc, AnyScope, DefaultScope, LockedObject
 import ..Dagger: order, dependents, noffspring, istask, inputs, unwrap_weak_checked, affinity, tochunk, timespan_start, timespan_finish, procs, move, chunktype, processor, default_enabled, get_processors, get_parent, execute!, rmprocs!, addprocs!, thunk_processor, constrain, cputhreadtime
-import ..Dagger: @dagdebug
+import ..Dagger: @dagdebug, @safe_lock_spin1
 import DataStructures: PriorityQueue, enqueue!, dequeue_pair!, peek
 
 import ..Dagger

Diff for: src/sch/util.jl

@@ -1,7 +1,7 @@
 "Like `errormonitor`, but tracks how many outstanding tasks are running."
 function errormonitor_tracked(t::Task)
     errormonitor(t)
-    lock(ERRORMONITOR_TRACKED) do tracked
+    @safe_lock_spin1 ERRORMONITOR_TRACKED tracked begin
         push!(tracked, t)
     end
     errormonitor(Threads.@spawn begin

Diff for: src/utils/locked-object.jl

@@ -14,3 +14,29 @@ function Base.lock(f, x::LockedObject)
         unlock(x.lock)
     end
 end
+Base.trylock(x::LockedObject) = trylock(x.lock)
+Base.unlock(x::LockedObject) = unlock(x.lock)
+payload(x::LockedObject) = x.payload
+
+# TODO: Move this back to MemPool
+# If we actually want to acquire a lock from a finalizer, we can't cause a task
+# switch. As a NonReentrantLock can only be taken by another thread that should
+# be running, and not a concurrent task we'd need to switch to, we can safely
+# spin.
+macro safe_lock_spin1(l, o, ex)
+    quote
+        temp = $(esc(l))
+        while !trylock(temp)
+            # we can't yield here
+            GC.safepoint()
+        end
+        MemPool.enable_finalizers(false) # retains compatibility with non-finalizer callers
+        try
+            $(esc(o)) = $payload(temp)
+            $(esc(ex))
+        finally
+            unlock(temp)
+            MemPool.enable_finalizers(true)
+        end
+    end
+end