From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751725AbcJJCDi (ORCPT <rfc822;w@1wt.eu>);
        Sun, 9 Oct 2016 22:03:38 -0400
Received: from mail-pa0-f65.google.com ([209.85.220.65]:35891 "EHLO
        mail-pa0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751441AbcJJCDh (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sun, 9 Oct 2016 22:03:37 -0400
Date: Sat, 8 Oct 2016 04:40:15 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
        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>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC][PATCHv2 3/7] printk: introduce per-cpu alt_print seq buffer
Message-ID: <20161007194015.GB3496@swordfish>
References: <20160930151758.8965-1-sergey.senozhatsky@gmail.com>
 <20160930151758.8965-4-sergey.senozhatsky@gmail.com>
 <20161001022415.GA527@swordfish>
 <20161006145656.GH13369@pathway.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161006145656.GH13369@pathway.suse.cz>
User-Agent: Mutt/1.7.1 (2016-10-04)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On (10/06/16 16:56), Petr Mladek wrote:
[..]
> > there are many WARN_ON_* on
> > vprintk_alt()->alt_printk_log_store()->vsnprintf() path but they all
> > seem to be calling printk():
> > 
> > - WARN_ON_ONCE() from vsnprintf()
> > - WARN_ONCE() from vsnprintf()->format_decode()
> > - WARN_ON vsnprintf()->set_field_width()
> > - WARN_ON from vsnprintf()->set_precision()
> > - WARN_ON from vsnprintf()->pointer()->flags_string()
> > 
> > a side note, some of these WARNs are... 'funny'. e.g., to deadlock a
> > system it's enough to just pass an unsupported flag in format string.
> > vsnprintf() will
> >     WARN_ONCE(1, "Please remove unsupported %%%c in format string\n", *fmt)
> > 
> > but the problem is that we are already in printk():
> > 
> >    printk()
> >     raw_spin_lock(&logbuf_lock)
> >     text_len = vscnprintf(text, sizeof(textbuf), fmt, args)
> >                 WARN_ONCE(1, "Please remove unsupported ...)
> >                  printk()
> >                   raw_spin_lock(&logbuf_lock)			<< deadlock
> 
> Just for record. This vscnprintf() is called when logbuf_cpu is set.
> Therefore this particular recursion is not possible at the moment.

ah, indeed. thanks.

> Anyway, I agree that alt_printk_enter() calls might get nested.
> The direct vprintk_emit() calls are one reason. I guess that
> we will need to use the same counter/enter functions also
> for WARN_*DEFERRED(). And it would cause nesting as well.
> 
> Therefore we need to be careful about loosing the original
> value of irq flags.
> 
> The question is whether we need to store the flags in
> a per-CPU variable. We might also store it on the stack
> of the enter()/exit() function caller. I mean something like

yes, let's keep it on the stack. this particular implementation
was just an experiment, and I hate it. what I currently have in
my tree:

#define alt_printk_enter(flags)                 \
        do {                                    \
                local_irq_save(flags);          \
                __alt_printk_enter();           \
        } while (0)

#define alt_printk_exit(flags)                  \
        do {                                    \
                __alt_printk_exit();            \
                local_irq_restore(flags);       \
        } while (0)

	-ss