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.
thanks
>
> -h
>
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