Date:	Tue, 16 Oct 2012 22:25:16 +0800
From:	Qing Z <njumical@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mingo@...nel.org, ben@...adent.org.uk, markivx@...eaurora.org,
	ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	cxie4@...vell.com, binw@...vell.com, wwang27@...vell.com,
	xjian@...vell.com, zhangwm@...vell.com, Qing Zhu <qzhu@...vell.com>
Subject: Re: [PATCH] panic: fix incomplete panic log in panic()

2012/10/16 Andrew Morton <akpm@...ux-foundation.org>:
> On Mon, 15 Oct 2012 19:38:46 +0800
> Qing Z <njumical@...il.com> wrote:
>
>> >>       atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
>> >>
>> >> +     /*
>> >> +      * Unlock the console anyway here, in case it's occupied by another
>> >> +      * one which has no chance to unlock the console thus prevents the
>> >> +      * panic log prints on the console.
>> >> +      */
>> >> +     console_unlock();
>> >> +
>> >>       bust_spinlocks(0);
>> >>
>> >>       if (!panic_blink)
>> >
>> > hm.  console_unlock() does a large amount of work, and it seems risky
>> > to do all of that when the system is in a bad-and-getting-worse state.
>> >
>> > Is there some more modest thing we could do here, for example,
>> >
>> >         if (console_locked) {
>> >                 up(&console_sem);
>> >                 console_locked = 0;
>> >         }
>> >
>> > or something along those lines?
>> >
>> > Also, perhaps this operation should be moved into bust_spinlocks().
>> > What would have happened if your code had triggered an oops, rather
>> > than called panic()?
>> >
>> >
>> Hi Andrew,
>>       Thanks for your reply!
>>       For your question" What would have happened if your code had
>> triggered an oops, rather than called panic()?", actually we found the
>> issue when trigger an oops. When we call FBIOPAN_DISPLAY in
>> ./drivers/video/fbmem.c, it will first lock console, if we trigger an
>> oops before unlock console, the issue happen. It also exist when call
>> panic() directly in the same case. It is a common issue for panic
>> process.
>>       I have two options for solution:
>>       1. I agree with your suggestion that add some modest thing in
>> bust_spinlocks(), bust_spinlocks() is supposed to clear spinlocks
>> which prevent oops information from reaching the user. But it didn't
>> clear console_sem. We can add codes that clear console_sem.
>>       1) add up(&console_sem) in bust_spinlocks(0).
>>               It will be risky in case that no printk after bust_spinlocks(0) in
>> panic(), because no console_unlock() to print log out.
>>       2) call console_unlock()in bust_spinlocks(0).
>>               For bust_spinlocks(0), console_unblank() is used to flush oops to
>> mtdoops console(commit: b61312d353da1871778711040464b10f5cd904df).
>> Logically, if panic without the issue, console_unlock is called after
>> couples of console_lock and console_unlock; if panic with the issue,
>> will it be risky call console_unlock() in console_unblank() after
>> console_lock()?
>>       2. Moreover, there is another option. We can also add protect codes
>> in vprintk(), vprintk() just cover the cases that two cores' log
>> interleave when panic and printk recurse itself. We can add all cases'
>> protection here. Actually the original vprintk() don___t have the issue,
>> but after the patch(commit: fe21773d655c2c64641ec2cef499289ea175c817)
>> which fix two cores' log interleave issue , the issue is not covered.
>> I add a flag after panic_smp_self_stop() in panic(), and check the
>> flag, if flag is set, vprintk will call zap_locks(), I have tested the
>> option, the issue also disappear.
>>       What do you think?
>
> The #1 priority is to get the oops message reliably delivered.
>
> That means we should avoid console_unlock() on the oops path: it's far
> too complicated and risks deadlocks, re-oopses, recursion, etc.
>
> If there was text queued in the console layer and that text fails to be
> emitted, well, that's sad, but it's more important that the oops
> message be displayed.
>
> If the oops trace is occasionally interleaved with other text then
> that's sad too, but at least the info we need is readable.  Oopses
> inside console_lock() are rare.
>
>
> So I'd suggest that the code in bust_spinlocks(1) should simply do
> whatever needs to be done to make the forthcoming oops trace be
> visible, and leave it at that - don't bother trying to flush out any
> old text.
>
>
> Also, we should be careful with things like up() on a semaphore which
> hasn't been down()ed.  Because under some Kconfig combinations, such an
> operation might trigger debugging traces and we could get into a big
> mess.  (An up() on non-down()ed semaphore is actually an OK operation,
> so this was a bad example.  But you see the problem).
>
Hi Andrew,
         It is indeed risky that call console_unlock() directly in
panic process, like deadlock that  core0 stop core1 but core1 is
holding logbuf_lock.
         As to old text, if we hold logbuf_lock when panic,we can keep
panic log continuous and entire, old text will be before panic
log.Frankly, I don't see any risk that old text impact or interleave
panic log.
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