From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: Marcos Paulo de Souza <mpdesouza@suse.com>,
Daniel Thompson <daniel@riscstar.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Steven Rostedt <rostedt@goodmis.org>,
Sergey Senozhatsky <senozhatsky@chromium.org>,
Jason Wessel <jason.wessel@windriver.com>,
Daniel Thompson <danielt@kernel.org>,
Douglas Anderson <dianders@chromium.org>,
linux-kernel@vger.kernel.org,
kgdb-bugreport@lists.sourceforge.net
Subject: Re: [PATCH v2 3/3] kdb: Adapt kdb_msg_write to work with NBCON consoles
Date: Mon, 01 Sep 2025 14:33:00 +0206 [thread overview]
Message-ID: <84plca5pez.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <aLWHmY9_I4rbV0wG@pathway.suse.cz>
On 2025-09-01, Petr Mladek <pmladek@suse.com> wrote:
> What about the following race?
>
> kdb_printf_cpu = -1 (0xffffffff)
>
> CPU 0xff CPU 0x1
>
> panic()
>
> printk()
> nbcon_atomic_flush_pending()
> nbcon_context_try_acquire_direct()
> # load low byte of kdb_printf_cpu
> val = 0xff
>
> vkdb_printf()
> cmpxchg(&kdb_printf_cpu, ...)
> kdb_printf_cpu == 0x1
>
> # load higher byte of kdb_printf_cpu
> val = 0xff
>
> Result: CPU 0xff would be allowed to acquire the nbcon context
> because it thinks that vkdb_printf() got locked on this CPU.
>
> It is not fully artificial, see
> https://lwn.net/Articles/793253/#Load%20Tearing
>
> The above race is not critical. CPU 0x1 still could wait for CPU 0xff
> and acquire the nbcon context later.
>
> But it is something unexpected. I would feel more comfortable if
> we used the READ_ONCE() and be on the safe side.
Agreed.
John
next prev parent reply other threads:[~2025-09-01 12:27 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-08-11 13:32 [PATCH v2 0/3] Handle NBCON consoles on KDB Marcos Paulo de Souza
2025-08-11 13:32 ` [PATCH v2 1/3] printk: nbcon: Export console_is_usable Marcos Paulo de Souza
2025-08-20 15:03 ` Petr Mladek
2025-08-11 13:32 ` [PATCH v2 2/3] printk: nbcon: Introduce KDB helpers Marcos Paulo de Souza
2025-08-26 14:30 ` John Ogness
2025-08-29 12:30 ` Marcos Paulo de Souza
2025-08-28 15:26 ` Petr Mladek
2025-08-29 12:33 ` Marcos Paulo de Souza
2025-08-11 13:32 ` [PATCH v2 3/3] kdb: Adapt kdb_msg_write to work with NBCON consoles Marcos Paulo de Souza
2025-08-11 14:38 ` Daniel Thompson
2025-08-11 19:19 ` Marcos Paulo de Souza
2025-08-26 15:11 ` John Ogness
2025-08-29 13:15 ` Petr Mladek
2025-08-29 14:01 ` Marcos Paulo de Souza
2025-08-29 14:12 ` John Ogness
2025-09-01 11:46 ` Petr Mladek
2025-09-01 12:27 ` John Ogness [this message]
2025-08-12 17:14 ` [PATCH v2 0/3] Handle NBCON consoles on KDB Marcos Paulo de Souza
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