linux-kernel - Re: [PATCHv7 6/8] printk: use printk

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Date:   Wed, 1 Feb 2017 17:15:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Jan Kara <jack@...e.cz>, Ross Zwisler <zwisler@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Calvin Owens <calvinowens@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Hurley <peter@...leysoftware.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCHv7 6/8] printk: use printk_safe buffers in printk

On Wed, Feb 01, 2017 at 04:39:10PM +0100, Petr Mladek wrote:
> I guess that you are talking about the introduction of
> #define SCHED_WARN_ON(x)	WARN_ONCE(x, #x)

No, there's a lot of regular WARN/WARN_ON/etc.. usage in the scheduler.
That thing was just a convenience wapper to print the condition that
warned.

> It reduces the risk of the deadlock but some risk is still there.
> IMHO, it does not avoid the lockdep warning.

It doesn't reduce anything, nor did it ever try. I really don't care if
it occasionally deadlocks, as long as it mostly gets out.

> One solution would be to hide the occasional deadlock and disable
> lockdep in SCHED_WARN_ON():
> 
> #define SCHED_WARN_ON(x)				\
> ({							\
> 	int __ret_sched_warn_on;			\
> 	lockdep_off();					\
> 	__ret_sched_warn_on = WARN_ONCE(x, #x);		\
> 	lockdep_on();					\
> 	unlikely(__ret_sched_warn_on);			\
> })

Like said, there's plenty of regular WARN/WARN_ON usage, so this will
not help much.

> Another solution would be to redirect it into the
> alternative buffer and let it printed later:
> 
> #define SCHED_WARN_ON(x)	WARN_ONCE(x, #x)		\
> ({								\
> 	unsigned long __sched_warn_on_flags;			\
> 	printk_safe_enter_irqsave(__sched_warn_on_flags);	\
> 	__ret_sched_warn_on = WARN_ONCE(x, #x);			\
> 	printk_safe_exit_irqrestore(__sched_warn_on_flags);	\
> 	unlikely(__ret_sched_warn_on);				\
> })

So my kernel doesn't yet have that abomination; that redirects it to a
buffer for later printing right? I hope that buffer is big enough to
hold a full WARN splat and the machine lives long enough to make it to
printing that crap.