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From: Sergey Senozhatsky <sergey.senozhatsky@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>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Hurley <peter@hurleysoftware.com>,
	linux-kernel@vger.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCHv6 5/7] printk: report lost messages in printk safe/nmi contexts
Date: Sat, 24 Dec 2016 00:08:27 +0900	[thread overview]
Message-ID: <20161223150827.GA7464@tigerII.localdomain> (raw)
In-Reply-To: <20161223105421.GC2541@linux.suse>

Hello,

On (12/23/16 11:54), Petr Mladek wrote:
[..]
> There is a potential race:
> 
> CPU0					CPU1
> 
> printk_safe_log_store()
>   len = atomic_read(&s->len);
> 
> 					__printk_save_flush()
> 
> 					  atomic_cmpxchg(&s->len, len, 0)
> 
> 					  report_message_lost(s);
> 
>    if (len >= sizeof(s->buffer) - 1) {
> 	atomic_inc(&s->message_lost);
> 		return 0;
> 
> We check the outdated len, account lost message, but it will not
> be reported until some other message appears in the log buffer.
> 
> > +
> >  out:
> 
> I would make sense to move report_message_lost(s) here, after
> the out: label.

hm, ok. to flush from another CPU we first need to have printk-safe/nmi
messages on that CPU, then return from printk-safe/nmi on that CPU, execute
per-CPU irq_wor, and then have concurrent printk-safe/nmi messages on current
CPU, in addition happening frequent enough to hit this case. I may be wrong,
but that's quite unlikely. I can move report_message_lost() to `out' label,
no problem. thanks for the report.

at some point I was actually considering turning ->message_lost into
'bool' -- "we lost your messages, we are sorry". the precise number of
lost messages doesn't help that much: the messages are gone, go and
increment CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT; that's all we can say now.

and speaking of lost messages. I think I found a regression in
console_unlock(). so I'll send out a fix ahead of this series.

and, besides, the logs I had a pleasure to look at today contained numerous
"%d printk messages dropped" with very accurate numbers, but those numbers
meant pretty much nothing to me - the messages were lost.

	-ss

  reply	other threads:[~2016-12-23 15:09 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-21 14:35 [PATCHv6 0/7] printk: use printk_safe to handle printk() recursive calls Sergey Senozhatsky
2016-12-21 14:35 ` [PATCHv6 1/7] printk: use vprintk_func in vprintk() Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 2/7] printk: rename nmi.c and exported api Sergey Senozhatsky
2016-12-21 19:45   ` Linus Torvalds
2016-12-22  1:17     ` Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 3/7] printk: introduce per-cpu safe_print seq buffer Sergey Senozhatsky
2016-12-22  0:53   ` kbuild test robot
2016-12-22  1:18     ` Sergey Senozhatsky
2016-12-22 16:36       ` Petr Mladek
2016-12-21 14:36 ` [PATCHv6 4/7] printk: always use deferred printk when flush printk_safe lines Sergey Senozhatsky
2016-12-21 14:36 ` [PATCHv6 5/7] printk: report lost messages in printk safe/nmi contexts Sergey Senozhatsky
2016-12-23 10:54   ` Petr Mladek
2016-12-23 15:08     ` Sergey Senozhatsky [this message]
2016-12-21 14:36 ` [PATCHv6 6/7] printk: use printk_safe buffers in printk Sergey Senozhatsky
2016-12-22  5:31   ` Sergey Senozhatsky
2016-12-22 17:10     ` Petr Mladek
2016-12-23  1:46       ` Sergey Senozhatsky
2016-12-23  9:53         ` Petr Mladek
2016-12-21 14:36 ` [PATCHv6 7/7] printk: remove zap_locks() function Sergey Senozhatsky

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