xfs deadlocks when preallocating files; say, from transmission starting a torrent; the "hung task" notifier first mentions xfsconvertd, then all that follows trying to write or sync the disk. i'm filing this bug as a placeholder for the moment as i didn't collect the kernel messages last time it occurred, but i plan on doing so as soon as possible.
Fixed upstream in commit: c626d174cfe38e7f0545d074c299527892cd8c45 xfs: prevent unwritten extent conversion from blocking I/O completion
oops, sorry, might have pulled the trigger to quickly. That's the commit that introduces xfsconvertd (I should look at the diffs first :/). Make sure you get the output of 'echo w > /proc/sysrq-trigger' when it next hangs...