linux-ext4 - Re: ext4 crash on 2.6.37: NULL ptr in ext4_discard

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Date:	Sun, 20 Feb 2011 09:39:23 -0500
From:	Mark Lord <kernel@...savvy.com>
To:	Ted Ts'o <tytso@....edu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: ext4 crash on 2.6.37: NULL ptr in ext4_discard_preallocations

On 11-02-20 08:55 AM, Mark Lord wrote:
> On 11-02-20 01:15 AM, Ted Ts'o wrote:
>> On Sun, Feb 20, 2011 at 12:05:27AM -0500, Mark Lord wrote:
>>> I suppose it must be, as there's no other 0x3c offset in that function.
>>> Which means it's probably this line that's crashing:
>>>
>>>              BUG_ON(pa->pa_obj_lock != &ei->i_prealloc_lock);
>>>
>>> ...which could only happen if "pa" was NULL there.
>>> I wonder how that happened ?
>>
>> Which could only happen if ei->i_prealloc_list were not properly
>> initialized (i..e, it was still NULL).  Which shouldn't ever
>> happen...., since all ext4_inodes are initialized in
>> ext4_alloc_inode().
>>
>> Hmm, can you replicate the crash?
> 
> So far it has been a one time deal here,
> but stuff like this is pretty serious nonetheless.
> 
> I suppose it could also happen if another thread did a list-delete
> at the same time as that function was running.  Which would require
> that there be a locking bug/confusion somewhere.
> 
> Looking over the code, most places use rcu to protect accesses,
> except for the fragment that crashed.  That's probably just fine,
> but something to reexamine just out of paranoia.
> 
> Also, the spinlock pointer appears to be dynamic, one of two
> possible spinlocks.  Maybe something got confused there
> (well, obviously *something* got confused, so..).

That looks like the best candidate:  perhaps pa->pa_obj_lock was
one of the per-cpu lg_prealloc_lock's at that point in time.
In which case an item could be deleted from the pa list
concurrently with the function that actually crashed?

That's as far as I can get with it in the time available.
You folks do know this code much better, so perhaps just expend
a few little grey cells on that theory before calling it quits?

Cheers!
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