diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index cfc2623273..ed50d32572 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -4621,6 +4621,15 @@ int libxl_domain_setmaxmem(libxl_ctx *ctx, uint32_t domid, uint32_t max_memkb)
     uint32_t memorykb;
     char *dompath = libxl__xs_get_dompath(gc, domid);
     int rc = 1;
+    libxl__domain_userdata_lock *lock = NULL;
+
+    CTX_LOCK;
+
+    lock = libxl__lock_domain_userdata(gc, domid);
+    if (!lock) {
+        rc = ERROR_LOCK_FAIL;
+        goto out;
+    }
 
     mem = libxl__xs_read(gc, XBT_NULL, libxl__sprintf(gc, "%s/memory/target", dompath));
     if (!mem) {
@@ -4647,6 +4656,8 @@ int libxl_domain_setmaxmem(libxl_ctx *ctx, uint32_t domid, uint32_t max_memkb)
 
     rc = 0;
 out:
+    if (lock) libxl__unlock_domain_userdata(lock);
+    CTX_UNLOCK;
     GC_FREE;
     return rc;
 }
@@ -4746,6 +4757,15 @@ int libxl_set_memory_target(libxl_ctx *ctx, uint32_t domid,
     libxl_dominfo ptr;
     char *uuid;
     xs_transaction_t t;
+    libxl__domain_userdata_lock *lock;
+
+    CTX_LOCK;
+
+    lock = libxl__lock_domain_userdata(gc, domid);
+    if (!lock) {
+        rc = ERROR_LOCK_FAIL;
+        goto out_no_transaction;
+    }
 
 retry_transaction:
     t = xs_transaction_start(ctx->xsh);
@@ -4867,6 +4887,8 @@ out:
             goto retry_transaction;
 
 out_no_transaction:
+    if (lock) libxl__unlock_domain_userdata(lock);
+    CTX_UNLOCK;
     GC_FREE;
     return rc;
 }
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index e96d6b57a5..5594b54b69 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -3600,6 +3600,7 @@ typedef struct {
     libxl__carefd *carefd;
     char *path; /* path of the lock file itself */
 } libxl__domain_userdata_lock;
+/* The CTX_LOCK must be held around uses of this lock */
 libxl__domain_userdata_lock *libxl__lock_domain_userdata(libxl__gc *gc,
                                                          uint32_t domid);
 void libxl__unlock_domain_userdata(libxl__domain_userdata_lock *lock);