Blocked tasks on 3.15.1

[Rich Freeman <r-btrfs@thefreemanclan.net>]

I've been getting blocked tasks on 3.15.1 generally at times when the
filesystem is somewhat busy (such as doing a backup via scp/clonezilla
writing to the disk).

A week ago I had enabled snapper for a day which resulted in a daily
cleanup of about 8 snapshots at once, which might have contributed,
but I've been limping along since.

Here is a pastebin of my dmesg from the hung tasks and a subsequent Alt-SysRq-W:

http://pastebin.com/yYdcxFTE

When this happens the system remains somewhat stable, but no writes to
the disk succeed, and I start getting load averages in the dozens as
tasks start blocking.

On reboot the system generally works fine, though it can hang a day or
two later.

I'm happy to try patches, or try to capture any other output that is
helpful the next time this happens - the system is fairly stable as
long as I capture things someplace other than my btrfs file systems.
I didn't see anything quite like this on the list.  I updated my
kernel around the time this behavior started, and was on 3.15.0
previously (though I haven't tried reverting yet).

Rich