diff --git a/scd/apdu.c b/scd/apdu.c
index 07df9fea9..a343307e7 100644
--- a/scd/apdu.c
+++ b/scd/apdu.c
@@ -367,7 +367,8 @@ unlock_slot (int slot)
 
 
 /* Find an unused reader slot for PORTSTR and put it into the reader
-   table.  Return -1 on error or the index into the reader table. */
+   table.  Return -1 on error or the index into the reader table.
+   Acquire slot's lock on successful return.  Caller needs to unlock it.  */
 static int
 new_reader_slot (void)
 {
@@ -394,6 +395,11 @@ new_reader_slot (void)
       reader_table[reader].lock_initialized = 1;
     }
 #endif /*USE_GNU_PTH*/
+  if (lock_slot (reader))
+    {
+      log_error ("error locking mutex: %s\n", strerror (errno));
+      return -1;
+    }
   reader_table[reader].connect_card = NULL;
   reader_table[reader].disconnect_card = NULL;
   reader_table[reader].close_reader = NULL;
@@ -675,6 +681,7 @@ open_ct_reader (int port)
       log_error ("apdu_open_ct_reader failed on port %d: %s\n",
                  port, ct_error_string (rc));
       reader_table[reader].used = 0;
+      unlock_slot (reader);
       return -1;
     }
 
@@ -696,6 +703,7 @@ open_ct_reader (int port)
   reader_table[reader].keypad_modify = NULL;
 
   dump_reader_status (reader);
+  unlock_slot (reader);
   return reader;
 }
 
@@ -1701,6 +1709,7 @@ open_pcsc_reader_direct (const char *portstr)
       log_error ("pcsc_establish_context failed: %s (0x%lx)\n",
                  pcsc_error_string (err), err);
       reader_table[slot].used = 0;
+      unlock_slot (slot);
       if (err == 0x8010001d)
         pcsc_no_service = 1;
       return -1;
@@ -1717,6 +1726,7 @@ open_pcsc_reader_direct (const char *portstr)
           log_error ("error allocating memory for reader list\n");
           pcsc_release_context (reader_table[slot].pcsc.context);
           reader_table[slot].used = 0;
+	  unlock_slot (slot);
           return -1 /*SW_HOST_OUT_OF_CORE*/;
         }
       err = pcsc_list_readers (reader_table[slot].pcsc.context,
@@ -1729,6 +1739,7 @@ open_pcsc_reader_direct (const char *portstr)
       pcsc_release_context (reader_table[slot].pcsc.context);
       reader_table[slot].used = 0;
       xfree (list);
+      unlock_slot (slot);
       return -1;
     }
 
@@ -1755,6 +1766,7 @@ open_pcsc_reader_direct (const char *portstr)
       log_error ("error allocating memory for reader name\n");
       pcsc_release_context (reader_table[slot].pcsc.context);
       reader_table[slot].used = 0;
+      unlock_slot (slot);
       return -1;
     }
   strcpy (reader_table[slot].rdrname, portstr? portstr : list);
@@ -1774,6 +1786,7 @@ open_pcsc_reader_direct (const char *portstr)
   reader_table[slot].dump_status_reader = dump_pcsc_reader_status;
 
   dump_reader_status (slot);
+  unlock_slot (slot);
   return slot;
 }
 #endif /*!NEED_PCSC_WRAPPER */
@@ -1821,6 +1834,7 @@ open_pcsc_reader_wrapped (const char *portstr)
     {
       log_error ("error creating a pipe: %s\n", strerror (errno));
       slotp->used = 0;
+      unlock_slot (slot);
       return -1;
     }
   if (pipe (wp) == -1)
@@ -1829,6 +1843,7 @@ open_pcsc_reader_wrapped (const char *portstr)
       close (rp[0]);
       close (rp[1]);
       slotp->used = 0;
+      unlock_slot (slot);
       return -1;
     }
 
@@ -1841,6 +1856,7 @@ open_pcsc_reader_wrapped (const char *portstr)
       close (wp[0]);
       close (wp[1]);
       slotp->used = 0;
+      unlock_slot (slot);
       return -1;
     }
   slotp->pcsc.pid = pid;
@@ -1976,6 +1992,7 @@ open_pcsc_reader_wrapped (const char *portstr)
   pcsc_get_status (slot, &dummy_status);
 
   dump_reader_status (slot);
+  unlock_slot (slot);
   return slot;
 
  command_failed:
@@ -1986,6 +2003,7 @@ open_pcsc_reader_wrapped (const char *portstr)
   kill (slotp->pcsc.pid, SIGTERM);
   slotp->pcsc.pid = (pid_t)(-1);
   slotp->used = 0;
+  unlock_slot (slot);
   /* There is no way to return SW. */
   return -1;
 
@@ -2422,6 +2440,7 @@ open_ccid_reader (const char *portstr)
   if (err)
     {
       slotp->used = 0;
+      unlock_slot (slot);
       return -1;
     }
 
@@ -2456,6 +2475,7 @@ open_ccid_reader (const char *portstr)
   reader_table[slot].is_t0 = 0;
 
   dump_reader_status (slot);
+  unlock_slot (slot);
   return slot;
 }
 
@@ -2694,6 +2714,7 @@ open_rapdu_reader (int portno,
   if (!slotp->rapdu.handle)
     {
       slotp->used = 0;
+      unlock_slot (slot);
       return -1;
     }
 
@@ -2748,12 +2769,14 @@ open_rapdu_reader (int portno,
 
   dump_reader_status (slot);
   rapdu_msg_release (msg);
+  unlock_slot (slot);
   return slot;
 
  failure:
   rapdu_msg_release (msg);
   rapdu_release (slotp->rapdu.handle);
   slotp->used = 0;
+  unlock_slot (slot);
   return -1;
 }