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Date: Mon, 26 Nov 2018 13:57:09 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCH v2 07/17] debugobjects: Move printk out of db lock
 critical sections
Message-ID: <20181126045709.GD540@jagdpanzerIV>
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To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, Peter Zijlstra <peterz@infradead.org>, Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>, Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, linux-mm@kvack.org, iommu@lists.linux-foundation.org, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Tejun Heo <tj@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Steven Rostedt <rostedt@goodmis.org>

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