diff --git a/lib/discover/cache.go b/lib/discover/cache.go
index ad1faf9b..60685880 100644
--- a/lib/discover/cache.go
+++ b/lib/discover/cache.go
@@ -26,7 +26,7 @@ type CachingMux struct {
 	*suture.Supervisor
 	finders []cachedFinder
 	caches  []*cache
-	mut     sync.Mutex
+	mut     sync.RWMutex
 }
 
 // A cachedFinder is a Finder with associated cache timeouts.
@@ -54,7 +54,7 @@ type cachedError interface {
 func NewCachingMux() *CachingMux {
 	return &CachingMux{
 		Supervisor: suture.NewSimple("discover.cachingMux"),
-		mut:        sync.NewMutex(),
+		mut:        sync.NewRWMutex(),
 	}
 }
 
@@ -75,7 +75,7 @@ func (m *CachingMux) Add(finder Finder, cacheTime, negCacheTime time.Duration, p
 func (m *CachingMux) Lookup(deviceID protocol.DeviceID) (direct []string, relays []Relay, err error) {
 	var pdirect []prioritizedAddress
 
-	m.mut.Lock()
+	m.mut.RLock()
 	for i, finder := range m.finders {
 		if cacheEntry, ok := m.caches[i].Get(deviceID); ok {
 			// We have a cache entry. Lets see what it says.
@@ -129,7 +129,7 @@ func (m *CachingMux) Lookup(deviceID protocol.DeviceID) (direct []string, relays
 			m.caches[i].Set(deviceID, entry)
 		}
 	}
-	m.mut.Unlock()
+	m.mut.RUnlock()
 
 	direct = uniqueSortedAddrs(pdirect)
 	relays = uniqueSortedRelays(relays)
@@ -149,12 +149,12 @@ func (m *CachingMux) Error() error {
 }
 
 func (m *CachingMux) ChildErrors() map[string]error {
-	m.mut.Lock()
 	children := make(map[string]error, len(m.finders))
+	m.mut.RLock()
 	for _, f := range m.finders {
 		children[f.String()] = f.Error()
 	}
-	m.mut.Unlock()
+	m.mut.RUnlock()
 	return children
 }
 
@@ -163,7 +163,7 @@ func (m *CachingMux) Cache() map[protocol.DeviceID]CacheEntry {
 	// children's caches.
 	res := make(map[protocol.DeviceID]CacheEntry)
 
-	m.mut.Lock()
+	m.mut.RLock()
 	for i := range m.finders {
 		// Each finder[i] has a corresponding cache at cache[i]. Go through it
 		// and populate the total, if it's newer than what's already in there.
@@ -183,7 +183,7 @@ func (m *CachingMux) Cache() map[protocol.DeviceID]CacheEntry {
 			}
 		}
 	}
-	m.mut.Unlock()
+	m.mut.RUnlock()
 
 	return res
 }
diff --git a/lib/discover/cache_test.go b/lib/discover/cache_test.go
index 0205e219..c7900c2f 100644
--- a/lib/discover/cache_test.go
+++ b/lib/discover/cache_test.go
@@ -91,3 +91,55 @@ func (f *fakeDiscovery) String() string {
 func (f *fakeDiscovery) Cache() map[protocol.DeviceID]CacheEntry {
 	return nil
 }
+
+func TestCacheSlowLookup(t *testing.T) {
+	c := NewCachingMux()
+	c.ServeBackground()
+	defer c.Stop()
+
+	// Add a slow discovery service.
+
+	started := make(chan struct{})
+	f1 := &slowDiscovery{time.Second, started}
+	c.Add(f1, time.Minute, 0, 0)
+
+	// Start a lookup, which will take at least a second
+
+	t0 := time.Now()
+	go c.Lookup(protocol.LocalDeviceID)
+	<-started // The slow lookup method has been called so we're inside the lock
+
+	// It should be possible to get ChildErrors while it's running
+
+	c.ChildErrors()
+
+	// Only a small amount of time should have passed, not the full second
+
+	diff := time.Since(t0)
+	if diff > 500*time.Millisecond {
+		t.Error("ChildErrors was blocked for", diff)
+	}
+}
+
+type slowDiscovery struct {
+	delay   time.Duration
+	started chan struct{}
+}
+
+func (f *slowDiscovery) Lookup(deviceID protocol.DeviceID) (direct []string, relays []Relay, err error) {
+	close(f.started)
+	time.Sleep(f.delay)
+	return nil, nil, nil
+}
+
+func (f *slowDiscovery) Error() error {
+	return nil
+}
+
+func (f *slowDiscovery) String() string {
+	return "fake"
+}
+
+func (f *slowDiscovery) Cache() map[protocol.DeviceID]CacheEntry {
+	return nil
+}