diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index 3c528a09..13a961d2 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -1917,11 +1917,21 @@
 
 /**
  * PPP_INPROC_MULTITHREADED==1 call ppp_input() using tcpip_callback().
- * Set this to 0 if pppos_input() is called inside tcpip_thread or with NO_SYS==1.
- * Default is 1 for NO_SYS==0 (multithreaded) and 0 for NO_SYS==1 (single-threaded).
+ *
+ * Set this to 0 in the following cases:
+ *  - pppos_input() is called from the main loop and NO_SYS==1
+ *  - you are using tcpip_input() (NO_SYS==0) on PPP data input
+ *
+ * Otherwise, if pppos_input() is called outside lwIP context (IRQ)
+ * set this to 1.
+ *
+ * CAUTION: if set to 1, you should NEVER call pppos_connect(), pppos_listen()
+ * and ppp_free() if pppos_input() can still be running, doing this is not
+ * thread safe. You should also avoid calling pppos_input() if PPPoS session
+ * is not started yet.
  */
 #ifndef PPP_INPROC_MULTITHREADED
-#define PPP_INPROC_MULTITHREADED (NO_SYS==0)
+#define PPP_INPROC_MULTITHREADED        0
 #endif
 
 /**
diff --git a/src/netif/ppp/pppos.c b/src/netif/ppp/pppos.c
index 2dce3403..bb4badb3 100644
--- a/src/netif/ppp/pppos.c
+++ b/src/netif/ppp/pppos.c
@@ -556,8 +556,14 @@ PACK_STRUCT_END
 #endif
 #endif /* PPP_INPROC_MULTITHREADED */
 
-/** Pass received raw characters to PPPoS to be decoded. This function is
- * thread-safe and can be called from a dedicated RX-thread or from a main-loop.
+/** Pass received raw characters to PPPoS to be decoded.
+ *
+ * This function is thread-safe if PPP_INPROC_MULTITHREADED is set to 1
+ * and can be called from a dedicated RX-thread or from interrupt context
+ * BUT you should NEVER call pppos_connect(), pppos_listen()
+ * and ppp_free() if pppos_input() can still be running, doing this is not
+ * thread safe. You should also avoid calling pppos_input() if PPPoS session
+ * is not started yet.
  *
  * @param pcb PPP descriptor index, returned by ppp_new()
  * @param data received data