serial: core: Document lock requirement for UPF_* flags updates

The flags field of struct uart_port can only be safely modified
if the port mutex is held; no other lock prevents concurrent
changes from corrupting the field.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index cf3a1e789bf5..8cb267b1fcd5 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -152,6 +152,7 @@ struct uart_port {
         unsigned long           sysrq;                  /* sysrq timeout */
 #endif
 
+        /* flags must be updated while holding port mutex */
         upf_t                   flags;
 
 #define UPF_FOURPORT            ((__force upf_t) (1 << 1))