Re: [PATCH] Btrfs: fix race when reusing stale extent buffers that leads to BUG_ON

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

On Thu, 23 Apr 2015 13:34:15 +0100, Filipe Manana wrote:

> On 04/23/2015 01:16 PM, Holger Hoffstätte wrote:
>> On Thu, 23 Apr 2015 11:28:48 +0100, Filipe Manana wrote:
>> 
>>> There's a race between releasing extent buffers that are flagged as stale
>>> and recycling them that makes us it the following BUG_ON at
>>> btrfs_release_extent_buffer_page:
>>>
>>>     BUG_ON(extent_buffer_under_io(eb))
>>>
>>> The BUG_ON is triggered because the extent buffer has the flag
>>> EXTENT_BUFFER_DIRTY set as a consequence of having been reused and made
>>> dirty by another concurrent task.
>> 
>> Awesome analysis!
>> 
>>> @@ -4768,6 +4768,25 @@ struct extent_buffer *find_extent_buffer(struct btrfs_fs_info *fs_info,
>>>  			       start >> PAGE_CACHE_SHIFT);
>>>  	if (eb && atomic_inc_not_zero(&eb->refs)) {
>>>  		rcu_read_unlock();
>>> +		/*
>>> +		 * Lock our eb's refs_lock to avoid races with
>>> +		 * free_extent_buffer. When we get our eb it might be flagged
>>> +		 * with EXTENT_BUFFER_STALE and another task running
>>> +		 * free_extent_buffer might have seen that flag set,
>>> +		 * eb->refs == 2, that the buffer isn't under IO (dirty and
>>> +		 * writeback flags not set) and it's still in the tree (flag
>>> +		 * EXTENT_BUFFER_TREE_REF set), therefore being in the process
>>> +		 * of decrementing the extent buffer's reference count twice.
>>> +		 * So here we could race and increment the eb's reference count,
>>> +		 * clear its stale flag, mark it as dirty and drop our reference
>>> +		 * before the other task finishes executing free_extent_buffer,
>>> +		 * which would later result in an attempt to free an extent
>>> +		 * buffer that is dirty.
>>> +		 */
>>> +		if (test_bit(EXTENT_BUFFER_STALE, &eb->bflags)) {
>>> +			spin_lock(&eb->refs_lock);
>>> +			spin_unlock(&eb->refs_lock);
>>> +		}
>>>  		mark_extent_buffer_accessed(eb, NULL);
>>>  		return eb;
>>>  	}
>> 
>> After staring at this (and the Lovecraftian horrors of free_extent_buffer())
>> for over an hour and trying to understand how and why this could even remotely
>> work, I cannot help but think that this fix would shift the race to the much
>> smaller window between the test_bit and the first spin_lock.
>> Essentially you subtly phase-shifted all participants and make them avoid the
>> race most of the time, yet I cannot help but think it's still there (just much
>> smaller), and could strike again with different scheduling intervals.
>> 
>> Would this be accurate?
> 
> Hi Holger,
> 
> Can you explain how the race can still happen?
> 
> The goal here is just to make sure a reader does not advance too fast if
> the eb is stale and there's a concurrent call to free_extent_buffer() in
> progress.

Yes, that much I understood. I look at this change from the perspective of
an optimizing compiler:

- without the new if-block we would fall through and mark_extent_buffer
while it's being deleted, which complains with a BUG.

- the new block therefore checks for the problematic state, but then does
something that - from a work perspective - could be eliminated (lock+unlock),
since nothing seemimgly useful happens inbetween.

-that leaves the if without observable side effect and so could be
eliminated as well.

So theoretically we have not really "coordinated" anything. That made me
suspicious: at the very least I would have expected some kind of loop
or something that protects/reliably delays mark_extent_buffer so that it
really has a completion/atomicity guarantee, not just a small "bump" in
its timeline. You said that a "real fix" would be proper refcounting -
that's sort of what I was expecting, at least in terms of side effects.

Now, I understand that the block is not eliminated, but the if followed
by the lock acquiry is not atomic. That's where - very theoretically -
the same race as before could happen e.g. on a single core and the thread
running free_extent_buffer is scheduled after the if but before the lock.
We'd then get the lock, immediately unlock it and proceed to mark.

I don't know if any of this can actually happen! It's just that I've never
seen a construct like this (and I like lock-free/wait-free coordination),
so this got me curious.

Thanks!
Holger