linux-kernel - Re: [PATCH] rtc: snvs: fix possible race condition

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Date:   Sat, 20 Jul 2019 21:55:51 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Trent Piepho <tpiepho@...inj.com>
Cc:     "linux-rtc@...r.kernel.org" <linux-rtc@...r.kernel.org>,
        "anson.huang@....com" <anson.huang@....com>,
        "a.zummo@...ertech.it" <a.zummo@...ertech.it>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "aisheng.dong@....com" <aisheng.dong@....com>,
        "linux-imx@....com" <linux-imx@....com>
Subject: Re: [PATCH] rtc: snvs: fix possible race condition

On 19/07/2019 19:04:21+0000, Trent Piepho wrote:
> On Fri, 2019-07-19 at 02:57 +0000, Anson Huang wrote:
> > 
> > > I do worry that handling the irq before the rtc device is registered could still
> > > result in a crash.  From what I saw, the irq path in snvs only uses driver state
> > > members that are fully initialized for the most part, and the allocated but
> > > unregistered data->rtc is only used in one call to rtc_update_irq(), which
> > > appears to be ok with this.
> > > 
> > > But it is not that hard to imagine that something could go into the rtc core
> > > that assumes call like rtc_update_irq() are only made on registered devices.
> > > 
> > > If there was a way to do it, I think allocating the irq in a masked state and
> > > then unmasking it as part of the final registration call to make the device go
> > > live would be a safer and more general pattern.
> > 
> > It makes sense, I think we can just move the devm_request_irq() to after rtc_register_device(),
> > It will make sure everything is ready before IRQ is enabled. Will send out a V2 patch. 
> 
> That will mean registering the rtc, then unregistering it if the irq
> request fails.  More of a pain to write this failure path.
> 
> Alexandre, is it part of rtc core design that rtc_update_irq() might be
> called on a rtc device that is properly allocated, but not registered
> yet?

Yes, the main reason of the change of API was exactly to handle this.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com