Re: [RFC 0/5] printk: Implement WARN_*DEFERRED()

From: Sergey Senozhatsky
Date: Tue Sep 27 2016 - 21:21:30 EST

On (09/27/16 18:02), Petr Mladek wrote:
> The main trick is that we replace the per-CPU function pointer
> by a preempt_count-like variable that could track the printk context.
> I know that Sergey has another ideas in this area. But I wanted to see
> how this approach would look like.

well, yes. I was looking at WARN_*_DEFERRED [1] for some time, and, I
think, the maintenance cost of that solution is just too high:

a) every existing WARN_* in sched/timekeeping/who knows where else
must be evaluated to ensure that in can't be called from printk()
path. if `false' - then the corresponding macro must be replaced
with _DEFERRED flavor.

b) any patch that adds new WARN_* usages must be additionally checked
to ensure that each of new WARN_* macros cannot be called from printk
path. if `false' -- the corresponding macro must be replaced with
_DEFERRED flavor.

c) any patch that refactors the code or moves some function calls around
etc. must be additionally checked for any accidental WARN_* from printk
path. even though if none of the patches added any new WARN_* to the code.

b) apart from WARN_* there can be `accidental' pr_err/pr_debug/etc. not
necessarily newly added (see 'c').

that's too much.
for example [not blaming anyone], a recent patch [2] that added a reasonable
WARN_ON_ONCE to assert_clock_updated() which, however, can result in a
possible printk() deadlock scenario that you, Petr, outlined [3]:

:+ printk()
: + vprintk_func -> vprintk_default()
: + vprinkt_emit()
: + console_unlock()
: + up_console_sem()
: + up() # takes &sem->lock
: + __up()
: + wake_up_process()
: + try_to_wake_up()
: + ttwu_queue()
: + ttwu_do_activate()
: + ttwu_do_wakeup()
: + rq_clock()
: + lockdep_assert_held()
: + printk()
: + vprintk_func -> vprintk_default()
: + vprintk_emit()
: + console_try_lock()
: + down_trylock_console_sem()
: + __down_trylock_console_sem()
: + down_trylock()

it takes a lot of additional effort, because both reviewer and contributor
must consider printk() internals. and, what's worse, if something goes
unnoticed we end up having a printk() deadlock again.

so I decided to address some of printk() issues in printk.c, not in
kernel/time/timekeeping.c or kernel/sched/core.c or anywhere else.

> Mid-air collision:
> I have just realized that Sergey sent another patchset that was
> more generic, complicated, and had some similarities, see

yeah, I should have Cc-ed a wider audience. do I need to resend the
patch set with the `extended' Cc list?