linux-kernel - Re: [PATCH] panic: move bust_spinlocks(0) after console_flush_on

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Message-ID: <20180604214423.3014dc63@vmware.local.home>
Date:   Mon, 4 Jun 2018 21:44:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Hoeun Ryu <hoeun.ryu@....com.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>, Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Hoeun Ryu <hoeun.ryu@....com>, linux-kernel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH] panic: move bust_spinlocks(0) after
 console_flush_on_panic() to avoid deadlocks

On Mon,  4 Jun 2018 14:45:57 +0900
Hoeun Ryu <hoeun.ryu@....com.com> wrote:

> From: Hoeun Ryu <hoeun.ryu@....com>
> 
>  Many console device drivers hold the uart_port->lock spinlock with irq enabled
> (using spin_lock()) while the device drivers are writing characters to their devices,
> but the device drivers just try to hold the spin lock (using spin_trylock()) if
> "oops_in_progress" is equal or greater than 1 to avoid deadlocks.
> 
>  There is a case ocurring a deadlock related to the lock and oops_in_progress. A CPU
> could be stopped by smp_send_stop() while it was holding the port lock because irq was
> enabled. Once a CPU stops, it doesn't respond interrupts anymore and the lock stays
> locked forever.
> 
>  console_flush_on_panic() is called during panic() and it eventually holds the uart
> lock but the lock is held by another stopped CPU and it is a deadlock. By moving
> bust_spinlocks(0) after console_flush_on_panic(), let the console device drivers
> think the Oops is still in progress to call spin_trylock() instead of spin_lock() to
> avoid the deadlock.
> 
> Signed-off-by: Hoeun Ryu <hoeun.ryu@....com>
> ---
>  kernel/panic.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 42e4874..b4063b6 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -233,8 +233,6 @@ void panic(const char *fmt, ...)
>  	if (_crash_kexec_post_notifiers)
>  		__crash_kexec(NULL);
>  
> -	bust_spinlocks(0);
> -
>  	/*
>  	 * We may have ended up stopping the CPU holding the lock (in
>  	 * smp_send_stop()) while still having some valuable data in the console
> @@ -246,6 +244,8 @@ void panic(const char *fmt, ...)
>  	debug_locks_off();
>  	console_flush_on_panic();
>  
> +	bust_spinlocks(0);

Added a few more to Cc. This looks like it could have subtle
side-effects. I'd like those that have been touching the code around
here to have a look.

-- Steve


> +
>  	if (!panic_blink)
>  		panic_blink = no_blink;
>