linux-kernel - Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock critical sections

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Date:   Mon, 26 Nov 2018 13:57:09 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock
 critical sections

On (11/23/18 12:48), Petr Mladek wrote:
[..]
> > This should make serial consoles re-entrant.
> > So printk->console_driver_write() hopefully will not deadlock.
> 
> Is the re-entrance safe? Some risk might be acceptable in Oops/panic
> situations. It is much less acceptable for random warnings.

Good question.

But what's the alternative? A deadlock in a serial console driver; such
that even panic() is not guaranteed to make through it (at least of now).
debug objects are used from the code which cannot re-entrant console
drivers.

bust_spinlock is called from various paths, not only panic.
git grep bust_spinlocks | wc -l
62

So we already switch to re-entrant consoles (and accept the risks) in
mm/fault.c, kernel/traps.c and so on. Which, I guess, makes us a little
more confident, faults/traps happen often enough.

It seems, that, more or less, serial consoles are ready to handle it.
UART consoles in ->write() callbacks just do a bunch of writel() [for
every char + \r\n].

	-ss