Dynamic reads without locking.

[Tim Kientzle]

> Bruce M Simpson wrote:
>>
>> Or keep a generation count to detect pre-emption (the devstat code does
>> this, amongst other things), and try again if you lost the race.

On further inspection, I'm pretty sure that sys/kern/subr_devstat.c
is not correct.

In particular, sysctl_devstat writes out a node's data without
holding the lock.  (I believe this is the whole point of all
the "generation count" machinery.)  It seems possible for
devstat_remove_entry to remove that entry, free the storage, and
have that storage recycled just before it gets written out.

In this particular case, it means random kernel data gets
written out from the kernel to userspace.  (Which in some circles
would be considered a security problem, though it's hard to see
how anyone could possibly exploit it.)

Tim Kientzle