On Wed 2020-06-03 12:52:15, Sumit Garg wrote:
> In kgdb context, calling console handlers aren't safe due to locks used
> in those handlers which could in turn lead to a deadlock. Although, using
> oops_in_progress increases the chance to bypass locks in most console
> handlers but it might not be sufficient enough in case a console uses
> more locks (VT/TTY is good example).
>
> Currently when a driver provides both polling I/O and a console then kdb
> will output using the console. We can increase robustness by using the
> currently active polling I/O driver (which should be lockless) instead
> of the corresponding console. For several common cases (e.g. an
> embedded system with a single serial port that is used both for console
> output and debugger I/O) this will result in no console handler being
> used.
>
> In order to achieve this we need to reverse the order of preference to
> use dbg_io_ops (uses polling I/O mode) over console APIs. So we just
> store "struct console" that represents debugger I/O in dbg_io_ops and
> while emitting kdb messages, skip console that matches dbg_io_ops
> console in order to avoid duplicate messages. After this change,
> "is_console" param becomes redundant and hence removed.
>
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index 4139698..6e182aa 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -558,6 +557,7 @@ static int __init kgdboc_earlycon_init(char *opt)
> }
>
> earlycon = con;
> + kgdboc_earlycon_io_ops.cons = con;
> pr_info("Going to register kgdb with earlycon '%s'\n", con->name);
> if (kgdb_register_io_module(&kgdboc_earlycon_io_ops) != 0) {
> earlycon = NULL;
Should we clear kgdboc_earlycon_io_ops.cons here when
kgdb_register_io_module() failed?
> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index c62d764..529116b 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -276,8 +276,7 @@ struct kgdb_arch {
> * the I/O driver.
> * @post_exception: Pointer to a function that will do any cleanup work
> * for the I/O driver.
> - * @is_console: 1 if the end device is a console 0 if the I/O device is
> - * not a console
> + * @cons: valid if the I/O device is a console; else NULL.
> */
> struct kgdb_io {
> const char *name;
> @@ -288,7 +287,7 @@ struct kgdb_io {
> void (*deinit) (void);
> void (*pre_exception) (void);
> void (*post_exception) (void);
> - int is_console;
> + struct console *cons;
Nit: I would call it "con". The trailing 's' makes me feel that that the
variable points to an array or list of consoles.
> };
>
Otherwise, it looks pretty straightforward.
Best Regards,
Petr
