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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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