diff --git a/Project.toml b/Project.toml
index 815e411..0f8e694 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "GPUToolbox"
 uuid = "096a3bc2-3ced-46d0-87f4-dd12716f4bfc"
-version = "3.3.1"
+version = "3.3.2"
 
 [deps]
 LLVM = "929cbde3-209d-540e-8aea-75f648917ca0"
diff --git a/src/synchronization.jl b/src/synchronization.jl
index 62e98b0..38c558b 100644
--- a/src/synchronization.jl
+++ b/src/synchronization.jl
@@ -195,13 +195,50 @@ end
 # loop to be available). the calling thread is adopted by Julia, which is safe as long as
 # driver calls that may be waiting on the callback are GC-safe.
 
-# a spin lock-based condition, so that signalling from a driver thread never blocks it in
-# Julia's scheduler (as a `ReentrantLock` could)
+# a spin lock whose waiters give up their CPU when they have to wait for long. the lock of a
+# completion is held by the driver's thread while it wakes up the waiting task, which
+# immediately takes the lock again. if that task runs on the same CPU, it preempts the
+# driver's thread, and would spin until the OS switches back. when all cores are busy (e.g.,
+# with PoCL executing the next kernel), that can take hundreds of µs.
+struct YieldingSpinLock <: Base.AbstractLock
+    lock::Threads.SpinLock
+end
+YieldingSpinLock() = YieldingSpinLock(Threads.SpinLock())
+
+const LOCK_SPIN_ITERATIONS = 100
+
+function Base.lock(l::YieldingSpinLock)
+    i = 0
+    while !trylock(l.lock)
+        if i < LOCK_SPIN_ITERATIONS
+            ccall(:jl_cpu_pause, Cvoid, ())
+            i += 1
+        else
+            yield_cpu()
+        end
+        GC.safepoint()
+    end
+    return
+end
+Base.trylock(l::YieldingSpinLock) = trylock(l.lock)
+Base.unlock(l::YieldingSpinLock) = unlock(l.lock)
+Base.islocked(l::YieldingSpinLock) = islocked(l.lock)
+Base.assert_havelock(l::YieldingSpinLock) = Base.assert_havelock(l.lock)
+
+# let the OS run another thread on this CPU
+@static if Sys.iswindows()
+    yield_cpu() = ccall((:SwitchToThread, "kernel32"), stdcall, Cint, ())
+else
+    yield_cpu() = ccall(:sched_yield, Cint, ())
+end
+
+# a condition protected by a spin lock, so that signalling from a driver thread never blocks
+# it in Julia's scheduler (as a `ReentrantLock` could)
 mutable struct Completion
-    const cond::Base.ThreadSynchronizer
+    const cond::Base.GenericCondition{YieldingSpinLock}
     @atomic state::Int      # one of the constants below
 
-    Completion() = new(Base.ThreadSynchronizer(), PENDING)
+    Completion() = new(Base.GenericCondition(YieldingSpinLock()), PENDING)
 end
 const PENDING = 0
 const SIGNALLED = 1
diff --git a/test/synchronization.jl b/test/synchronization.jl
index 1720f10..2e1dd8f 100644
--- a/test/synchronization.jl
+++ b/test/synchronization.jl
@@ -333,3 +333,67 @@ end
     GC.gc(); GC.gc()
     @test isassigned(ret) && ret[] == Some(:waited)
 end
+
+# run `f` on a thread that is not managed by Julia
+mutable struct ForeignCall
+    const f::Any
+    @atomic done::Bool
+    @atomic failed::Bool
+end
+const foreign_calls = ForeignCall[]     # keep them alive
+function foreign_call(arg::Ptr{Cvoid})
+    call = unsafe_pointer_to_objref(arg)::ForeignCall
+    try
+        call.f()
+    catch
+        @atomic call.failed = true
+    end
+    @atomic call.done = true
+    return
+end
+function on_foreign_thread(f)
+    call = ForeignCall(f, false, false)
+    push!(foreign_calls, call)
+    tid = Ref{NTuple{32, UInt8}}(ntuple(_ -> 0x0, 32))
+    cb = @cfunction(foreign_call, Cvoid, (Ptr{Cvoid},))
+    err = ccall(:uv_thread_create, Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}),
+                tid, cb, pointer_from_objref(call))
+    @test err == 0
+    ccall(:uv_thread_detach, Cint, (Ptr{Cvoid},), tid)
+    return call
+end
+
+@testset "completion lock" begin
+    # a task that is woken up while the notifying thread still holds the lock waits for it
+    # to be released
+    cond = Base.GenericCondition(GPUToolbox.YieldingSpinLock())
+    signalled = Ref(false)
+    waiter = @async @lock cond begin
+        while !signalled[]
+            wait(cond)
+        end
+        islocked(cond)
+    end
+    call = on_foreign_thread() do
+        @lock cond begin
+            signalled[] = true
+            notify(cond)
+            @gcsafe_ccall uv_sleep(10::Cuint)::Cvoid
+        end
+    end
+    @test timedwait(() -> istaskdone(waiter), 30) === :ok
+    @test fetch(waiter)
+    @test timedwait(() -> @atomic(call.done), 30) === :ok
+    @test !@atomic(call.failed)
+
+    # a driver thread waiting for the lock lets the garbage collector run
+    c = GPUToolbox.Completion()
+    GC.@preserve c begin
+        lock(c.cond)
+        signal_later(pointer_from_objref(c), 0)
+        @gcsafe_ccall uv_sleep(10::Cuint)::Cvoid
+        GC.gc()
+        unlock(c.cond)
+        @test timedwait(() -> (@atomic c.state) == GPUToolbox.RELEASED, 30) === :ok
+    end
+end