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Date: Wed, 27 May 2020 14:31:15 +0100
From: Daniel Thompson <daniel.thompson@...aro.org>
To: Sumit Garg <sumit.garg@...aro.org>
Cc: kgdb-bugreport@...ts.sourceforge.net, jason.wessel@...driver.com,
dianders@...omium.org, pmladek@...e.com,
sergey.senozhatsky@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] kdb: Switch kdb_msg_write() to use safer polling
I/O
On Wed, May 27, 2020 at 11:55:59AM +0530, Sumit Garg wrote:
> In kgdb NMI context, calling console handlers isn't safe due to locks
> used in those handlers which could 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.
Not sure I would have predicted all those changes to kgdboc.c based on
this patch description. I assume this is to help identify which console
matches our dbg_io_ops but it would be good to spell this out.
> Suggested-by: Daniel Thompson <daniel.thompson@...aro.org>
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> ---
> drivers/tty/serial/kgdboc.c | 17 ++++++++---------
> include/linux/kgdb.h | 2 ++
> kernel/debug/kdb/kdb_io.c | 46 +++++++++++++++++++++++++++++++--------------
> 3 files changed, 42 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index c9f94fa..6199fe1 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -35,7 +35,6 @@ static struct kparam_string kps = {
> };
>
> static int kgdboc_use_kms; /* 1 if we use kernel mode switching */
> -static struct tty_driver *kgdb_tty_driver;
> static int kgdb_tty_line;
>
> #ifdef CONFIG_KDB_KEYBOARD
> @@ -154,7 +153,7 @@ static int configure_kgdboc(void)
> }
>
> kgdboc_io_ops.is_console = 0;
> - kgdb_tty_driver = NULL;
> + kgdboc_io_ops.tty_drv = NULL;
>
> kgdboc_use_kms = 0;
> if (strncmp(cptr, "kms,", 4) == 0) {
> @@ -178,7 +177,7 @@ static int configure_kgdboc(void)
> }
> }
>
> - kgdb_tty_driver = p;
> + kgdboc_io_ops.tty_drv = p;
> kgdb_tty_line = tty_line;
>
> do_register:
> @@ -216,18 +215,18 @@ static int __init init_kgdboc(void)
>
> static int kgdboc_get_char(void)
> {
> - if (!kgdb_tty_driver)
> + if (!kgdboc_io_ops.tty_drv)
> return -1;
> - return kgdb_tty_driver->ops->poll_get_char(kgdb_tty_driver,
> - kgdb_tty_line);
> + return kgdboc_io_ops.tty_drv->ops->poll_get_char(kgdboc_io_ops.tty_drv,
> + kgdb_tty_line);
> }
>
> static void kgdboc_put_char(u8 chr)
> {
> - if (!kgdb_tty_driver)
> + if (!kgdboc_io_ops.tty_drv)
> return;
> - kgdb_tty_driver->ops->poll_put_char(kgdb_tty_driver,
> - kgdb_tty_line, chr);
> + kgdboc_io_ops.tty_drv->ops->poll_put_char(kgdboc_io_ops.tty_drv,
> + kgdb_tty_line, chr);
> }
>
> static int param_set_kgdboc_var(const char *kmessage,
> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index b072aeb..05d165d 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -275,6 +275,7 @@ struct kgdb_arch {
> * 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
> + * @tty_drv: Pointer to polling tty driver.
> */
> struct kgdb_io {
> const char *name;
> @@ -285,6 +286,7 @@ struct kgdb_io {
> void (*pre_exception) (void);
> void (*post_exception) (void);
> int is_console;
> + struct tty_driver *tty_drv;
Should this be a struct tty_driver or a struct console?
In other words if the lifetime the console structure is the same as the
tty_driver then isn't it better to capture the console instead
(easier to compare and works with non-tty devices such as the
USB debug mode).
> };
>
> extern const struct kgdb_arch arch_kgdb_ops;
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index f848482..c2efa52 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -24,6 +24,7 @@
> #include <linux/kgdb.h>
> #include <linux/kdb.h>
> #include <linux/kallsyms.h>
> +#include <linux/tty_driver.h>
> #include "kdb_private.h"
>
> #define CMD_BUFLEN 256
> @@ -542,13 +543,18 @@ static int kdb_search_string(char *searched, char *searchfor)
> return 0;
> }
>
> -static void kdb_io_write(char *cp, int len, void (*io_put_char)(u8 ch))
> +static void kdb_io_write(char *cp, int len, void (*io_put_char)(u8),
> + struct tty_driver *p, int line,
> + void (*poll_put_char)(struct tty_driver *, int, char))
Judging from your reply to comment 1 I guess this is already on the list
to eliminate ;-).
Daniel.
> {
> if (len <= 0)
> return;
>
> while (len--) {
> - io_put_char(*cp);
> + if (io_put_char)
> + io_put_char(*cp);
> + if (poll_put_char)
> + poll_put_char(p, line, *cp);
> cp++;
> }
> }
> @@ -561,22 +567,34 @@ static void kdb_msg_write(char *msg, int msg_len)
> return;
>
> if (dbg_io_ops && !dbg_io_ops->is_console)
> - kdb_io_write(msg, msg_len, dbg_io_ops->write_char);
> + kdb_io_write(msg, msg_len, dbg_io_ops->write_char,
> + NULL, 0, NULL);
>
> for_each_console(c) {
> + int line;
> + struct tty_driver *p;
> +
> if (!(c->flags & CON_ENABLED))
> continue;
> - /*
> - * While rounding up CPUs via NMIs, its possible that
> - * a rounded up CPU maybe holding a console port lock
> - * leading to kgdb master CPU stuck in a deadlock during
> - * invocation of console write operations. So in order
> - * to avoid such a deadlock, enable oops_in_progress
> - * prior to invocation of console handlers.
> - */
> - ++oops_in_progress;
> - c->write(c, msg, msg_len);
> - --oops_in_progress;
> +
> + p = c->device ? c->device(c, &line) : NULL;
> + if (p && dbg_io_ops && p == dbg_io_ops->tty_drv && p->ops &&
> + p->ops->poll_put_char) {
> + kdb_io_write(msg, msg_len, NULL, p, line,
> + p->ops->poll_put_char);
> + } else {
> + /*
> + * While rounding up CPUs via NMIs, its possible that
> + * a rounded up CPU maybe holding a console port lock
> + * leading to kgdb master CPU stuck in a deadlock during
> + * invocation of console write operations. So in order
> + * to avoid such a deadlock, enable oops_in_progress
> + * prior to invocation of console handlers.
> + */
> + ++oops_in_progress;
> + c->write(c, msg, msg_len);
> + --oops_in_progress;
> + }
> touch_nmi_watchdog();
> }
> }
> --
> 2.7.4
>
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