Re: [tip:locking/urgent] compiler, atomics: Provide READ_ONCE_NOCHECK ()

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Wed, Oct 14, 2015 at 10:48 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>> > I'd still rather find a way to just delete get_wchan, but whatever.
>>
>> :-)
>
> AFAICS can only do that at the price of slowing down various scheduler functions
> by saving the caller address.
>

A quick check on my machine:

# for i in /proc/*/wchan; do cat $i && echo ''; done |sort |uniq

doesn't have very much of interest to say:

0
devtmpfsd
dmcrypt_write
do_sigtimedwait
do_wait
ep_poll
fsnotify_mark_destroy
futex_wait_queue_me
hrtimer_nanosleep
irq_thread
kauditd_thread
khugepaged
kjournald2
ksm_scan_thread
kswapd
kthreadd
pipe_wait
poll_schedule_timeout
rcu_gp_kthread
rcu_nocb_kthread
rescuer_thread
scsi_error_handler
sk_wait_data
smpboot_thread_fn
unix_stream_recvmsg
wait_woken
worker_thread
xfsaild

A bunch of those look like they're specific to kernel threads, for
which this whole mechanism is probably pointless -- just reading
/proc/PID/stack (with suitable privilege) is probably better.

For the rest, a few are useful, but I find myself wondering whether
this mechanism is really useful enough to be worth keeping.  We could
just return "asleep" in /proc/PID/wchan.

A more interesting thing to do might be to try to decode regs->orig_ax
to give a guess as to which syscall is asleep.  But this seems like a
lot of fiddling and a lot of worry about security issues for a
mechanism of dubious value.  Also:

$ cat /proc/1/wchan
ep_poll

Why do we allow unprivileged queries like that at all?

--Andy
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