diff --git a/libraries/AP_Filesystem/AP_Filesystem_FATFS.cpp b/libraries/AP_Filesystem/AP_Filesystem_FATFS.cpp
index b46c53a4ef..45a7ef8772 100644
--- a/libraries/AP_Filesystem/AP_Filesystem_FATFS.cpp
+++ b/libraries/AP_Filesystem/AP_Filesystem_FATFS.cpp
@@ -815,6 +815,8 @@ bool AP_Filesystem::set_mtime(const char *filename, const time_t mtime_sec)
     fno.fdate = fdate;
     fno.ftime = ftime;
 
+    WITH_SEMAPHORE(sem);
+
     return f_utime(filename, (FILINFO *)&fno) == FR_OK;
 }