linux-kernel - Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

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Date:   Thu, 10 Nov 2016 09:20:35 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, linux-usb@...r.kernel.org,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

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