diff --git a/base/locks-mt.jl b/base/locks-mt.jl
index 1efafe2822065..6a3b68016cb81 100644
--- a/base/locks-mt.jl
+++ b/base/locks-mt.jl
@@ -87,6 +87,7 @@ function trylock(l::SpinLock)
 end
 
 function unlock(l::SpinLock)
+    _get(l) == 0 && error("unlock count must match lock count")
     _set!(l, 0)
     GC.enable_finalizers(true)
     ccall(:jl_cpu_wake, Cvoid, ())
diff --git a/src/gc.c b/src/gc.c
index cf4c00f710633..c991e38296577 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -411,7 +411,12 @@ JL_DLLEXPORT void jl_gc_enable_finalizers(jl_ptls_t ptls, int on)
         }
         JL_CATCH {
             jl_printf((JL_STREAM*)STDERR_FILENO, "WARNING: GC finalizers already enabled on this thread. Possibly unpaired within a try block?\n");
-            jlbacktrace(); // written to STDERR_FILENO
+            // Only print the backtrace once, to avoid spamming the logs
+            static int backtrace_printed = 0;
+            if (backtrace_printed == 0) {
+                backtrace_printed = 1;
+                jlbacktrace(); // written to STDERR_FILENO
+            }
         }
         return;
     }
diff --git a/test/misc.jl b/test/misc.jl
index dd8ad47bc9129..33faa4b09eee8 100644
--- a/test/misc.jl
+++ b/test/misc.jl
@@ -126,7 +126,7 @@ end
 # Debugging tool: return the current state of the enable_finalizers counter.
 enable_finalizers_count() = ccall(:jl_gc_get_enable_finalizers, Int32, (Ptr{Cvoid},), C_NULL)
 
-let l = ReentrantLock()
+for l in (Threads.SpinLock(), ReentrantLock())
     @test enable_finalizers_count() == 0
     @test lock(enable_finalizers_count, l) == 1
     @test enable_finalizers_count() == 0
diff --git a/test/threads_exec.jl b/test/threads_exec.jl
index 691fca2fb2afa..c21ef45389f96 100644
--- a/test/threads_exec.jl
+++ b/test/threads_exec.jl
@@ -112,10 +112,10 @@ test_threaded_atomic_minmax(UInt16(27000),UInt16(37000))
 function threaded_add_locked(::Type{LockT}, x, n) where LockT
     critical = LockT()
     @threads for i = 1:n
-        @test lock(critical) === nothing
+        lock(critical)
         @test islocked(critical)
         x = x + 1
-        @test unlock(critical) === nothing
+        unlock(critical)
     end
     @test !islocked(critical)
     nentered = 0
@@ -124,7 +124,7 @@ function threaded_add_locked(::Type{LockT}, x, n) where LockT
         if trylock(critical)
             @test islocked(critical)
             nentered += 1
-            @test unlock(critical) === nothing
+            unlock(critical)
         else
             atomic_add!(nfailed, 1)
         end
@@ -142,21 +142,21 @@ end
 let critical = ReentrantLock()
     @test !islocked(critical)
     @test_throws ErrorException("unlock count must match lock count") unlock(critical)
-    @test lock(critical) === nothing
+    lock(critical)
     @test islocked(critical)
-    @test lock(critical) === nothing
-    @test trylock(critical) == true
+    lock(critical)
+    t = trylock(critical); @test t
     @test islocked(critical)
-    @test unlock(critical) === nothing
+    unlock(critical)
     @test islocked(critical)
-    @test unlock(critical) === nothing
+    unlock(critical)
     @test islocked(critical)
-    @test unlock(critical) === nothing
+    unlock(critical)
     @test !islocked(critical)
     @test_throws ErrorException("unlock count must match lock count") unlock(critical)
-    @test trylock(critical) == true
+    t = trylock(critical); @test t
     @test islocked(critical)
-    @test unlock(critical) === nothing
+    unlock(critical)
     @test !islocked(critical)
     @test_throws ErrorException("unlock count must match lock count") unlock(critical)
     @test !islocked(critical)
@@ -167,10 +167,10 @@ end
 function threaded_gc_locked(::Type{LockT}) where LockT
     critical = LockT()
     @threads for i = 1:20
-        @test lock(critical) === nothing
+        lock(critical)
         @test islocked(critical)
         GC.gc(false)
-        @test unlock(critical) === nothing
+        unlock(critical)
     end
     @test !islocked(critical)
 end