linux-kernel - Re: [RFC 0/5] printk: Implement WARN

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Date:   Thu, 29 Sep 2016 13:28:53 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>,
        Byungchul Park <byungchul.park@....com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Jan Kara <jack@...e.cz>, Luca Abeni <luca.abeni@...tn.it>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Yuyang Du <yuyang.du@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Tejun Heo <tj@...nel.org>, Calvin Owens <calvinowens@...com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC 0/5] printk: Implement WARN_*DEFERRED()

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,
Petr