diff --git a/src/calibre/utils/lock.py b/src/calibre/utils/lock.py
index 5090c11cf8..5d42d8a297 100644
--- a/src/calibre/utils/lock.py
+++ b/src/calibre/utils/lock.py
@@ -8,7 +8,7 @@ Secure access to locked files from multiple processes.
 
 from calibre.constants import iswindows, __appname__, \
                               win32api, win32event, winerror, fcntl
-import time, atexit, os, stat
+import time, atexit, os, stat, errno
 
 class LockError(Exception):
     pass
@@ -26,7 +26,7 @@ class WindowsExclFile(object):
                 self._handle = w.CreateFile(path,
                     w.GENERIC_READ|w.GENERIC_WRITE,  # Open for reading and writing
                     0,  # Open exclusive
-                    None,  # No security attributes
+                    None,  # No security attributes, ensures handle is not inherited by children
                     w.OPEN_ALWAYS,  # If file does not exist, create it
                     w.FILE_ATTRIBUTE_NORMAL,  # Normal attributes
                     None,  # No template file
@@ -39,7 +39,7 @@ class WindowsExclFile(object):
                 else:
                     raise
         if not hasattr(self, '_handle'):
-            raise Exception('Failed to open exclusive file: %s' % path)
+            raise LockError('Failed to open exclusive file: %s' % path)
 
     def seek(self, amt, frm=0):
         import win32file as w
@@ -108,8 +108,26 @@ class WindowsExclFile(object):
 def unix_open(path):
     # We cannot use open(a+b) directly because Fedora apparently ships with a
     # broken libc that causes seek(0) followed by truncate() to not work for
-    # files with O_APPEND set.
-    fd = os.open(path, os.O_RDWR | os.O_CREAT, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
+    # files with O_APPEND set. We also use O_CLOEXEC when it is available,
+    # to ensure there are no races.
+    flags = os.O_RDWR | os.O_CREAT
+    from calibre.constants import plugins
+    speedup = plugins['speedup'][0]
+    has_cloexec = False
+    if hasattr(speedup, 'O_CLOEXEC'):
+        try:
+            fd = os.open(path, flags | speedup.O_CLOEXEC, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
+            has_cloexec = True
+        except EnvironmentError as err:
+            if getattr(err, 'errno', None) == errno.EINVAL:  # Kernel does not support O_CLOEXEC
+                fd = os.open(path, flags, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
+            else:
+                raise
+    else:
+        fd = os.open(path, flags, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
+
+    if not has_cloexec:
+        fcntl.fcntl(fd, fcntl.F_SETFD, fcntl.FD_CLOEXEC)
     return os.fdopen(fd, 'r+b')
 
 class ExclusiveFile(object):
@@ -125,7 +143,7 @@ class ExclusiveFile(object):
         if not iswindows:
             while self.timeout < 0 or timeout >= 0:
                 try:
-                    fcntl.lockf(self.file.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
+                    fcntl.flock(self.file.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
                     break
                 except IOError:
                     time.sleep(1)
@@ -138,6 +156,31 @@ class ExclusiveFile(object):
     def __exit__(self, type, value, traceback):
         self.file.close()
 
+def test_exclusive_file(path=None):
+    if path is None:
+        import tempfile
+        f = os.path.join(tempfile.gettempdir(), 'test-exclusive-file')
+        with ExclusiveFile(f):
+            # Try same process lock
+            try:
+                with ExclusiveFile(f, timeout=1):
+                    raise LockError("ExclusiveFile failed to prevent multiple uses in the same process!")
+            except LockError:
+                pass
+            # Try different process lock
+            from calibre.utils.ipc.simple_worker import fork_job
+            err = fork_job('calibre.utils.lock', 'test_exclusive_file', (f,))['result']
+            if err is not None:
+                raise LockError('ExclusiveFile failed with error: %s' % err)
+    else:
+        try:
+            with ExclusiveFile(path, timeout=1):
+                raise Exception('ExclusiveFile failed to prevent multiple uses in different processes!')
+        except LockError:
+            pass
+        except Exception as err:
+            return str(err)
+
 def _clean_lock_file(file):
     try:
         file.close()
diff --git a/src/calibre/utils/speedup.c b/src/calibre/utils/speedup.c
index b05d8ee3c1..cf86298916 100644
--- a/src/calibre/utils/speedup.c
+++ b/src/calibre/utils/speedup.c
@@ -2,6 +2,7 @@
 #include <Python.h>
 
 #include <stdlib.h>
+#include <fcntl.h>
 
 #define min(x, y) ((x < y) ? x : y)
 #define max(x, y) ((x > y) ? x : y)
@@ -116,4 +117,7 @@ initspeedup(void) {
     "Implementation of methods in C for speed."
     );
     if (m == NULL) return;
+#ifdef O_CLOEXEC
+    PyModule_AddIntConstant(m, "O_CLOEXEC", O_CLOEXEC);
+#endif
 }