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 S1754926AbcKJBUl (ORCPT <rfc822;w@1wt.eu>);
        Wed, 9 Nov 2016 20:20:41 -0500
Received: from mga06.intel.com ([134.134.136.31]:43231 "EHLO mga06.intel.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1752824AbcKJBUj (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 9 Nov 2016 20:20:39 -0500
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.31,616,1473145200"; 
   d="scan'208";a="29576152"
Subject: Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability
To: Thomas Gleixner <tglx@linutronix.de>
References: <1477976354-13291-1-git-send-email-baolu.lu@linux.intel.com>
 <1477976354-13291-2-git-send-email-baolu.lu@linux.intel.com>
 <alpine.DEB.2.20.1611091030171.3501@nanos>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, linux-usb@vger.kernel.org,
        x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
From: Lu Baolu <baolu.lu@linux.intel.com>
Message-ID: <5823CB63.7090603@linux.intel.com>
Date: Thu, 10 Nov 2016 09:20:35 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.5.1
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.20.1611091030171.3501@nanos>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Hi,

On 11/09/2016 05:37 PM, Thomas Gleixner wrote:
> On Tue, 1 Nov 2016, Lu Baolu wrote:
>> +static void early_xdbc_write(struct console *con, const char *str, u32 n)
>> +{
>> +	int chunk, ret;
>> +	static char buf[XDBC_MAX_PACKET];
>> +	int use_cr = 0;
>> +
>> +	if (!xdbc.xdbc_reg)
>> +		return;
>> +	memset(buf, 0, XDBC_MAX_PACKET);
> How is that dealing with reentrancy?
>
> early_printk() does not protect against it. Peter has a patch to prevent
> concurrent access from different cpus, but it cannot and will never prevent
> reentrancy on the same cpu (interrupt, nmi).

I can use a spinlock_irq to protect reentrancy of interrupt on the same
cpu. But I have no idea about the nmi one. This seems to be a common
issue for all early printk drivers.

Peter, any idea?

Best regards,
Lu Baolu