linux-kernel - Re: [PATCH 2/2] printk: make sure to print log on console.

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Message-ID: <20180601091823.GA1841@jagdpanzerIV>
Date:   Fri, 1 Jun 2018 18:18:23 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Maninder Singh <maninder1.s@...sung.com>,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, a.sahrawat@...sung.com,
        pankaj.m@...sung.com, v.narang@...sung.com
Subject: Re: [PATCH 2/2] printk: make sure to print log on console.

On (06/01/18 10:53), Petr Mladek wrote:
> [...]
> 
> > So I'd say that most likely the following scenarios can suffer:
> > 
> > - NMI comes in, sets loglevel to X, printk-s some data, restores the
> >   loglevel back to Y
> > - IRQ comes in [like sysrq, etc] comes in and does the same thing
> > - software exception comes in and does the same thing [e.g. bust_spinlocks()
> >   at arch/s390/mm/fault.c]
> 
> 
> My view is:
> 
> The race with another printk() (console_lock owner) is much more
> likely than a race between two CPUs manipulating console_loglevel.

The race with console_loglevel manipulation from another CPU was not
the main point [it is unlikely, like I said in my "nitpick"].
The point was

	NMI / printk_safe section

	saved_console_loglevel = console_loglevel
	console_loglevel = A

	printk
	printk
	printk

	console_loglevel = saved_console_loglevel
	iret

Is not handled.

> The proposed patch seems to be in the right direction. It is supposed
> to fix the most likely scenario.

Could be.

> I am personally fine with this partial solution for now. We could
> always make it better if people meet the other scenarios.

I don't have objections. But I'd prefer to see real uses cases and
to know why partial solution is good enough in this case, even though
we know that NMI / printk_safe() messages may be lost due to very same
problem.

	-ss