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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html