Re: [PATCH] Btrfs: hold the tree mod lock in __tree_mod_log_rewind

[Josef Bacik <jbacik@fusionio.com>]

On Sun, Jun 30, 2013 at 10:25:05AM +0200, Jan Schmidt wrote:
> On 30.06.2013 05:17, Josef Bacik wrote:
> > We need to hold the tree mod log lock in __tree_mod_log_rewind since we walk
> > forward in the tree mod entries, otherwise we'll end up with random entries and
> > trip the BUG_ON() at the front of __tree_mod_log_rewind.  This fixes the panics
> > people were seeing when running
> > 
> > find /whatever -type f -exec btrfs fi defrag {} \;
> 
> This patch cannot help to solve the problem, as far as I've understood
> what is going on. It does change timing, though, which presumably makes
> it pass the current reproducer we're having.
> 
> On rewinding, iteration through the tree mod log rb-tree goes backwards
> in time, which means that once we've found our staring point we cannot
> be trapped by later additions. The old items we're rewinding towards
> cannot be freed, because we've allocated a blocker element within the
> tree and rewinding never goes beyond the allocated blocker. The blocker
> element is allocated by btrfs_get_tree_mod_seq and mostly referred to as
> time_seq within the other tree mod log functions in ctree.c. To sum up,
> the added lock is not required.
> 
> The debug output I've analyzed so far shows that after we've rewinded
> all REMOVE_WHILE_FREEING operations on a buffer, ordered consecutively
> as expected, there comes another REMOVE_WHILE_FREEING with a sequence
> number much further in the past for the same buffer (but that sequence
> number is still higher than out time_seq rewind barrier at that point).
> This must be a logical problem I've not completely understood so far,
> but locking doesn't seem to be the right track.
> 

Finally reproduced it, this is my output

 btrfs-endio-wri-23110 [000] ...2  9556.882103: __tree_mod_log_rewind: rewinding 15450537984
 btrfs-endio-wri-23110 [000] ...2  9556.882104: __tree_mod_log_rewind: 15450537984: processing ffff880246590a40, op 3, seq 68719476829, slot 0
 btrfs-endio-wri-23110 [000] ...2  9556.882106: __tree_mod_log_rewind: 15450537984: processing ffff880246590ac0, op 3, seq 68719476828, slot 1
 btrfs-endio-wri-23110 [000] ...2  9556.882108: __tree_mod_log_rewind: 15450537984: processing ffff880246590a40, op 3, seq 68719476829, slot 0
 btrfs-endio-wri-23110 [000] ...2  9556.882110: __tree_mod_log_rewind: 15450537984: this tm is failing, ffff880246590a40, seq 68719476829, slot 0

so I'm inclined to beleive I've got it right.  Thanks,

Josef