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

From: Petr Mladek
Date: Thu Sep 29 2016 - 07:29:11 EST

On Wed 2016-09-28 10:18:45, Sergey Senozhatsky wrote:
> 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.
> 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.

I see the point.

Your approach (alt buffer) adds some complexity to the printk code but
it allows to remove printk_deferred()/WARN_DEFERRED() and all the risk
of it. I am going to look closely on it.

Best Regards,