Re: [PATCH] Btrfs: remove transaction from send

[Josef Bacik <jbacik@fb.com>]

On 03/14/2014 09:13 AM, Wang Shilong wrote:
>> Lets try this again.  We can deadlock the box if we send on a box and try to
>> write onto the same fs with the app that is trying to listen to the send pipe.
>> This is because the writer could get stuck waiting for a transaction commit
>> which is being blocked by the send.  So fix this by making sure looking at the
>> commit roots is always going to be consistent.  We do this by keeping track of
>> which roots need to have their commit roots swapped during commit, and then
>> taking the commit_root_sem and swapping them all at once.  Then make sure we
>> take a read lock on the commit_root_sem in cases where we search the commit root
>> to make sure we're always looking at a consistent view of the commit roots.
>> Previously we had problems with this because we would swap a fs tree commit root
>> and then swap the extent tree commit root independently which would cause the
>> backref walking code to screw up sometimes.  With this patch we no longer
>> deadlock and pass all the weird send/receive corner cases.  Thanks,
> 
> Now btrfs send are alway searching commit root! Your codes only seems to protect backref codes,
> it reduce transaction blocked but make it not safe as we have discussed before.
> 
>

I was trying to remember why we didn't like this solution before but I
couldn't come up with anything.  Apparently I haven't completely fixed
the problem yet so stay tuned for what I do next ;).  Thanks,

Josef