From: Daniel Thompson <daniel.thompson@linaro.org>
To: Sumit Garg <sumit.garg@linaro.org>
Cc: kgdb-bugreport@lists.sourceforge.net, jason.wessel@windriver.com,
dianders@chromium.org, pmladek@suse.com,
sergey.senozhatsky@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/4] kdb: Make kdb_printf robust to run in NMI context
Date: Wed, 27 May 2020 15:26:17 +0100 [thread overview]
Message-ID: <20200527142617.t7dzal7ok5i3exra@holly.lan> (raw)
In-Reply-To: <1590560759-21453-4-git-send-email-sumit.garg@linaro.org>
On Wed, May 27, 2020 at 11:55:58AM +0530, Sumit Garg wrote:
> While rounding up CPUs via NMIs, its possible that a rounded up CPU
This problem does not just impact NMI roundup (breakpoints, including
implicit breakpoint-on-oops can have the same effect).
> 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.
>
> Suggested-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> ---
> kernel/debug/kdb/kdb_io.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 349dfcc..f848482 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -566,7 +566,17 @@ static void kdb_msg_write(char *msg, int msg_len)
> for_each_console(c) {
> if (!(c->flags & CON_ENABLED))
> continue;
> + /*
> + * While rounding up CPUs via NMIs, its possible that
Ditto.
> + * 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.
Actually looking at this comment as a whole I think it spends to many
words on what and not enough on why (e.g. what the tradeoffs are and
why we are not using bust_spinlocks() which would be a more obvious
approach).
Set oops_in_progress to encourage the console drivers to disregard
their internal spin locks: in the current calling context
the risk of deadlock is a bigger problem than risks due to
re-entering the console driver. We operate directly on
oops_in_progress rather than using bust_spinlocks() because
the calls bust_spinlocks() makes on exit are not appropriate
for this calling context.
Daniel.
> + */
> + ++oops_in_progress;
> c->write(c, msg, msg_len);
> + --oops_in_progress;
> touch_nmi_watchdog();
> }
> }
> --
> 2.7.4
>
next prev parent reply other threads:[~2020-05-27 14:26 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-27 6:25 [PATCH v3 0/4] kdb: Improve console handling Sumit Garg
2020-05-27 6:25 ` [PATCH v3 1/4] kdb: Re-factor kdb_printf() message write code Sumit Garg
2020-05-27 8:29 ` Daniel Thompson
2020-05-27 10:01 ` Sumit Garg
2020-05-27 6:25 ` [PATCH v3 2/4] kdb: Check status of console prior to invoking handlers Sumit Garg
2020-05-27 14:26 ` Daniel Thompson
2020-05-27 6:25 ` [PATCH v3 3/4] kdb: Make kdb_printf robust to run in NMI context Sumit Garg
2020-05-27 14:26 ` Daniel Thompson [this message]
2020-05-28 7:42 ` Sumit Garg
2020-05-27 6:25 ` [PATCH v3 4/4] kdb: Switch kdb_msg_write() to use safer polling I/O Sumit Garg
2020-05-27 13:31 ` Daniel Thompson
2020-05-28 6:18 ` Sumit Garg
2020-05-28 11:26 ` Daniel Thompson
2020-05-28 14:57 ` Petr Mladek
2020-05-29 5:46 ` Sumit Garg
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