diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 816d695a5..1f1468d97 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -16,6 +16,7 @@
 #include "machines.hh"
 #include "daemon.hh"
 #include "worker-protocol.hh"
+#include "cgroup.hh"
 
 #include <algorithm>
 #include <iostream>
@@ -2400,14 +2401,13 @@ void DerivationGoal::startBuilder()
             auto hostCgroup = canonPath("/sys/fs/cgroup/" + name + "/" + cgroup);
 
             if (!pathExists(hostCgroup))
-                throw Error("expected unified cgroup directory '%s'", hostCgroup);
+                throw Error("expected cgroup directory '%s'", hostCgroup);
 
             auto childCgroup = fmt("%s/nix-%d", hostCgroup, buildUser->getUID());
 
-            // FIXME: if the cgroup already exists, kill all processes
-            // in it and destroy it.
+            destroyCgroup(childCgroup);
 
-            if (mkdir(childCgroup.c_str(), 0755) == -1 && errno != EEXIST)
+            if (mkdir(childCgroup.c_str(), 0755) == -1)
                 throw SysError("creating cgroup '%s'", childCgroup);
 
             chownToBuilder(childCgroup);
diff --git a/src/libstore/cgroup.cc b/src/libstore/cgroup.cc
new file mode 100644
index 000000000..8cd682e68
--- /dev/null
+++ b/src/libstore/cgroup.cc
@@ -0,0 +1,49 @@
+#if __linux__
+
+#include "cgroup.hh"
+#include "util.hh"
+
+#include <chrono>
+
+#include <dirent.h>
+
+namespace nix {
+
+void destroyCgroup(const Path & cgroup)
+{
+    for (auto & entry : readDirectory(cgroup)) {
+        if (entry.type != DT_DIR) continue;
+        destroyCgroup(cgroup + "/" + entry.name);
+    }
+
+    int round = 1;
+
+    while (true) {
+        auto pids = tokenizeString<std::vector<std::string>>(readFile(cgroup + "/cgroup.procs"));
+
+        if (pids.empty()) break;
+
+        if (round > 20)
+            throw Error("cannot kill cgroup '%s'", cgroup);
+
+        for (auto & pid_s : pids) {
+            pid_t pid;
+            if (!string2Int(pid_s, pid)) throw Error("invalid pid '%s'", pid);
+            // FIXME: pid wraparound
+            if (kill(pid, SIGKILL) == -1 && errno != ESRCH)
+                throw SysError("killing member %d of cgroup '%s'", pid, cgroup);
+        }
+
+        auto sleep = std::chrono::milliseconds((int) std::pow(2.0, std::min(round, 10)));
+        printError("waiting for %d ms for cgroup '%s' to become empty", sleep.count(), cgroup);
+        std::this_thread::sleep_for(sleep);
+        round++;
+    }
+
+    if (rmdir(cgroup.c_str()) == -1)
+        throw SysError("deleting cgroup '%s'", cgroup);
+}
+
+}
+
+#endif
diff --git a/src/libstore/cgroup.hh b/src/libstore/cgroup.hh
new file mode 100644
index 000000000..c7b09398e
--- /dev/null
+++ b/src/libstore/cgroup.hh
@@ -0,0 +1,13 @@
+#pragma once
+
+#if __linux__
+
+#include "types.hh"
+
+namespace nix {
+
+void destroyCgroup(const Path & cgroup);
+
+}
+
+#endif