Various people have reported deadlocks and it has aways seemed a bit risky
to try to sync the filesystems at this stage anyway.

"I have seen panic failing two times lately on an SMP system. The box
 panic'ed but was running happily on the other cpus. The culprit of this
 failure is the fact, that these panics have been caused by a block device 
 or a filesystem (e.g. using errors=panic). In these cases the  likelihood 
 of a failure/hang of  sys_sync() is high. This is exactly what happened in
 both cases I have seen. Meanwhile the other cpus are happily continuing
 destroying data as the kernel has a severe problem but its not aware of
 that as smp_send_stop happens after sys_sync."
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>