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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jan Kara <jack@suse.cz>, Tejun Heo <tj@kernel.org>,
	Calvin Owens <calvinowens@fb.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Mel Gorman <mgorman@techsingularity.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Laura Abbott <labbott@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [RFC][PATCHv4 4/6] printk: report lost messages in printk safe/nmi contexts
Date: Thu, 1 Dec 2016 11:10:42 +0900	[thread overview]
Message-ID: <20161201021005.GE12039@jagdpanzerIV> (raw)
In-Reply-To: <20161125110730.GH24103@pathway.suse.cz>

On (11/25/16 12:07), Petr Mladek wrote:
[..]
> > +static void report_message_lost(atomic_t *num_lost, char *fmt)
> > +{
> > +	int lost = atomic_xchg(num_lost, 0);
> > +
> > +	if (lost) {
> > +		char msg[56];
> 
> I would really like to avoid a hard coded buffer size. Such things
> are likely to bite us in the future.

why would scnprintf() overflow.

> I thought about reshuffling a lot of logic, adding more wrappers,
> ... But the solution might be easy in the end, see below.
> 
> > +
> > +		scnprintf(msg, sizeof(msg), fmt, lost);
> > +
> > +		printk_safe_flush_line(msg, strlen(msg));
> 
> This made my brain spin a lot. I wondered if it did what we wanted
> and it was safe.
> 
> On one hand, it is supposed to work because use exactly this
> function in __printk_safe_flush() where you call this from.
> 
> One question is if it does what we want in different contexts.
> Let's look at it:
> 
> 1. It calls printk_deferred() in NMI context. There is a risk
>    of a deadlock. But it is called only from
>    printk_safe_flush_on_panic() which is the last resort. Therefore
>    it does exactly what we want.
> 
> 2. It calls printk()->printk_func()->vprintk_emit() in normal context.
>    It is what we want in normal context.
> 
> 3. It calls printk()->printk_func()->v printk_safe() in printk_safe
>    context. This does not look correct. IMHO, this might happen
>    only printk_safe_flush_on_panic() and we want to use
>    printk_deferred() here as well.
[..]
> The easiest solution would be to simply call printk_deferred()
> here. Everything will be deferred after the async printk() patchset
> anyway.
> 
> I would even use printk_deferred() in printk_safe_flush_line()
> for each context. It is not optimal but it works very well
> and it makes the code much more straightforward.

yes, good point.
we can call deferred printk for anything there; or replace that in_nmi()
check with the `printk_safe_context != 0' one, and then route the message
via printk or printk_deferred.


[..]
> >   * Flush data from the associated per-CPU buffer. The function
> >   * can be called either via IRQ work or independently.
> > @@ -147,6 +183,9 @@ static void __printk_safe_flush(struct irq_work *work)
> >  
> >  	i = 0;
> >  more:
> > +	report_nmi_message_lost();
> > +	report_safe_message_lost();
> 
> Please, move this at the end of this function.

ok.

	-ss

  reply	other threads:[~2016-12-01  2:10 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-27 15:49 [RFC][PATCHv4 0/6] printk: use printk_safe to handle printk() recursive calls Sergey Senozhatsky
2016-10-27 15:49 ` [RFC][PATCHv4 1/6] printk: use vprintk_func in vprintk() Sergey Senozhatsky
2016-11-24 16:28   ` Petr Mladek
2016-10-27 15:49 ` [RFC][PATCHv4 2/6] printk: rename nmi.c and exported api Sergey Senozhatsky
2016-11-24 16:35   ` Petr Mladek
2016-12-01  1:07     ` Sergey Senozhatsky
2016-12-01 12:12       ` Petr Mladek
2016-10-27 15:49 ` [RFC][PATCHv4 3/6] printk: introduce per-cpu safe_print seq buffer Sergey Senozhatsky
2016-11-24 16:58   ` Petr Mladek
2016-12-01  1:08     ` Sergey Senozhatsky
2016-12-01  5:32     ` Sergey Senozhatsky
2016-10-27 15:49 ` [RFC][PATCHv4 4/6] printk: report lost messages in printk safe/nmi contexts Sergey Senozhatsky
2016-11-25 11:07   ` Petr Mladek
2016-12-01  2:10     ` Sergey Senozhatsky [this message]
2016-12-01 12:50       ` Petr Mladek
2016-10-27 15:49 ` [RFC][PATCHv4 5/6] printk: use printk_safe buffers Sergey Senozhatsky
2016-11-25 14:28   ` Petr Mladek
2016-12-01  2:14     ` Sergey Senozhatsky
2016-10-27 15:49 ` [RFC][PATCHv4 6/6] printk: remove zap_locks() function Sergey Senozhatsky
2016-11-25 15:01   ` Petr Mladek
2016-11-25 15:17     ` Peter Zijlstra
2016-12-01  2:34       ` Sergey Senozhatsky
2016-12-01  5:42         ` Peter Zijlstra
2016-12-01 13:36           ` Petr Mladek
2016-12-02  1:11             ` Sergey Senozhatsky
2016-12-01  2:18     ` Sergey Senozhatsky
2016-12-01 12:50     ` Sergey Senozhatsky
2016-12-01 13:15       ` Petr Mladek
2016-10-28  3:30 ` [RFC][PATCHv4 0/6] printk: use printk_safe to handle printk() recursive calls Linus Torvalds
2016-10-28  4:05   ` Sergey Senozhatsky

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