tty/serial: do not free trasnmit buffer page under port lock
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>
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@ -205,10 +205,15 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
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if (!state->xmit.buf) {
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state->xmit.buf = (unsigned char *) page;
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uart_circ_clear(&state->xmit);
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uart_port_unlock(uport, flags);
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} else {
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uart_port_unlock(uport, flags);
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/*
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* Do not free() the page under the port lock, see
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* uart_shutdown().
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*/
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free_page(page);
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}
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uart_port_unlock(uport, flags);
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retval = uport->ops->startup(uport);
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if (retval == 0) {
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@ -268,6 +273,7 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
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struct uart_port *uport = uart_port_check(state);
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struct tty_port *port = &state->port;
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unsigned long flags = 0;
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char *xmit_buf = NULL;
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/*
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* Set the TTY IO error marker
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@ -298,14 +304,18 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
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tty_port_set_suspended(port, 0);
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/*
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* Free the transmit buffer page.
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* Do not free() the transmit buffer page under the port lock since
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* this can create various circular locking scenarios. For instance,
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* console driver may need to allocate/free a debug object, which
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* can endup in printk() recursion.
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*/
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uart_port_lock(state, flags);
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if (state->xmit.buf) {
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free_page((unsigned long)state->xmit.buf);
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state->xmit.buf = NULL;
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}
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xmit_buf = state->xmit.buf;
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state->xmit.buf = NULL;
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uart_port_unlock(uport, flags);
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if (xmit_buf)
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free_page((unsigned long)xmit_buf);
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}
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/**
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