4.20-stable review patch. If anyone has any objections, please let me know.
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[ Upstream commit d72402145ace0697a6a9e8e75a3de5bf3375f78d ]
LKP has hit yet another circular locking dependency between uart console drivers and debugobjects [1]:
CPU0 CPU1
rhltable_init() __init_work() debug_object_init uart_shutdown() /* db->lock */ /* uart_port->lock */ debug_print_object() free_page() printk() call_console_drivers() debug_check_no_obj_freed() /* uart_port->lock */ /* db->lock */ debug_print_object()
So there are two dependency chains: uart_port->lock -> db->lock And db->lock -> uart_port->lock
This particular circular locking dependency can be addressed in several ways:
a) One way would be to move debug_print_object() out of db->lock scope and, thus, break the db->lock -> uart_port->lock chain. b) Another one would be to free() transmit buffer page out of db->lock in UART code; which is what this patch does.
It makes sense to apply a) and b) independently: there are too many things going on behind free(), none of which depend on uart_port->lock.
The patch fixes transmit buffer page free() in uart_shutdown() and, additionally, in uart_port_startup() (as was suggested by Dmitry Safonov).
[1] https://lore.kernel.org/lkml/20181211091154.GL23332@shao2-debian/T/#u Signed-off-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Reviewed-by: Petr Mladek pmladek@suse.com Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Jiri Slaby jslaby@suse.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Waiman Long longman@redhat.com Cc: Dmitry Safonov dima@arista.com Cc: Steven Rostedt rostedt@goodmis.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/tty/serial/serial_core.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index c439a5a1e6c0..d4cca5bdaf1c 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -205,10 +205,15 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state, if (!state->xmit.buf) { state->xmit.buf = (unsigned char *) page; uart_circ_clear(&state->xmit); + uart_port_unlock(uport, flags); } else { + uart_port_unlock(uport, flags); + /* + * Do not free() the page under the port lock, see + * uart_shutdown(). + */ free_page(page); } - uart_port_unlock(uport, flags);
retval = uport->ops->startup(uport); if (retval == 0) { @@ -268,6 +273,7 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state) struct uart_port *uport = uart_port_check(state); struct tty_port *port = &state->port; unsigned long flags = 0; + char *xmit_buf = NULL;
/* * Set the TTY IO error marker @@ -298,14 +304,18 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state) tty_port_set_suspended(port, 0);
/* - * Free the transmit buffer page. + * Do not free() the transmit buffer page under the port lock since + * this can create various circular locking scenarios. For instance, + * console driver may need to allocate/free a debug object, which + * can endup in printk() recursion. */ uart_port_lock(state, flags); - if (state->xmit.buf) { - free_page((unsigned long)state->xmit.buf); - state->xmit.buf = NULL; - } + xmit_buf = state->xmit.buf; + state->xmit.buf = NULL; uart_port_unlock(uport, flags); + + if (xmit_buf) + free_page((unsigned long)xmit_buf); }
/**