linux-kernel - Re: [RFC PATCH] serial: core: Use pm_runtime_get_sync() in uart

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87il5o32w9.fsf@jogness.linutronix.de>
Date:   Sun, 26 Nov 2023 17:23:26 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Xuewen Yan <xuewen.yan@...soc.com>, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, ilpo.jarvinen@...ux.intel.com,
        tony@...mide.com
Cc:     tglx@...utronix.de, andriy.shevchenko@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        ke.wang@...soc.com, xuewen.yan94@...il.com,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH] serial: core: Use pm_runtime_get_sync() in
 uart_start()

[Added printk maintainers CC.]

On 2023-11-24, Xuewen Yan <xuewen.yan@...soc.com> wrote:
> The commit 84a9582fd203("serial: core: Start managing serial
> controllers to enable runtime PM") use the pm_runtime_get() after
> uart_port_lock() which would close the irq and disable preement.  At
> this time, pm_runtime_get may cause the following two problems:
>
> (1) deadlock in try_to_wake_up:
>
> uart_write()
> uart_port_lock() <<< get lock
> __uart_start
> __pm_runtime_resume
> rpm_resume
> queue_work_on
> try_to_wake_up
> _printk
> uart_console_write
> ...
> uart_port_lock() <<< wait forever

I suppose you got this because of the lockdep message generated by
#2. It probably would make sense to call __printk_safe_enter() inside
uart_port_lock(). This would allow printk() to automatically defer the
printing for that CPU until the port lock is released.

> (2) scheduling while atomic:
> uart_write()
> uart_port_lock() <<< get lock
> __uart_start
> __pm_runtime_resume
> rpm_resume
> schedule() << sleep

rpm_resume() is a fascinating function. It requires the caller to hold a
spin_lock (dev->power.lock) with interrupts disabled. But it seems to
believe that this is the *only* spin_lock held so that it can
temporarily spin_unlock and call might_sleep() functions. In the case of
uart_write(), it certainly is not the only spin_lock held.

I do not know enough about the internals of RPM to suggest a proper
solution. But it looks like rpm_resume() cannot assume dev->power.lock
is the only spin_lock held by the caller.

John Ogness