linux-kernel - Re: [PATCH] panic: release stale console lock to always get the logbuf printed out

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Message-ID: <87612hfziq.fsf@vitty.brq.redhat.com>
Date:	Thu, 08 Oct 2015 12:03:25 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jiri Kosina <jkosina@...e.cz>, Baoquan He <bhe@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Xie XiuQi <xiexiuqi@...wei.com>,
	Seth Jennings <sjenning@...hat.com>,
	linux-kernel@...r.kernel.org,
	"K. Y. Srinivasan" <kys@...rosoft.com>
Subject: Re: [PATCH] panic: release stale console lock to always get the logbuf printed out

Jan Kara <jack@...e.cz> writes:

>> --- a/kernel/panic.c~panic-release-stale-console-lock-to-always-get-the-logbuf-printed-out-fix
>> +++ a/kernel/panic.c
>> @@ -149,10 +149,10 @@ void panic(const char *fmt, ...)
>>  	bust_spinlocks(0);
>>  
>>  	/*
>> -	 * We may have ended up killing the CPU holding the lock and still have
>> -	 * some valuable data in console buffer. Try to acquire the lock and
>> -	 * release it regardless of the result. The release will also print the
>> -	 * buffers out.
>> +	 * We may have ended up stopping the CPU holding the lock (in
>> +	 * smp_send_stop()) while still having some valuable data in the console
>> +	 * buffer.  Try to acquire the lock then release it regardless of the
>> +	 * result.  The release will also print the buffers out.
>>  	 */
>>  	console_trylock();
>>  	console_unlock();
>> _
>> 
>> 
>> Does the console_trylock() guarantee that the console lock is now held?
>> If the console_lock-holding CPU is still running then there's a window
>> where the above code could enter console_unlock() when nobody's holding
>> console_lock.  If smp_send_stop() always works (synchronously) then
>> that won't happen.
>
> We have this mechanism using zap_locks() in kernel/printk/printk.c when
> crash happens on the CPU holding console_sem. Can't we use the same
> mechanism for this case? Something like adding:
>
> 	zap_locks();
> 	console_lock();
> 	console_unlock();
>
> to panic?

I thought it doesn't really matter who holds the lock at this point --
all other cpus were already stopped with IPIs/NMIs.

> If we picked up patch "kernel: Avoid softlockups in
> stop_machine() during heavy printing" from my series (it's completely
> independent, I've attached the latest version), the result would look less
> hacky to me (attached). Thoughts?

Haven't tested it but should also work...

Thanks,

-- 
  Vitaly
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