linux-kernel - Re: [PATCH] hw_random: atmel-rng: fix race condition leading to repeated bits

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Message-ID: <4FC214E5.1010007@atmel.com>
Date:	Sun, 27 May 2012 13:49:57 +0200
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	Peter Korsgaard <jacmet@...site.dk>
CC:	<herbert@...dor.apana.org.au>, <jamie@...ieiles.com>,
	<linux-kernel@...r.kernel.org>, <GPontis@...com>,
	<stable@...r.kernel.org>
Subject: Re: [PATCH] hw_random: atmel-rng: fix race condition leading to repeated
 bits

On 05/25/2012 12:10 PM, Peter Korsgaard :
>>>>>> "Nicolas" == Nicolas Ferre <nicolas.ferre@...el.com> writes:
> 
> Hi,
> 
>  Nicolas> What about a single read to ISR like this:
> 
>  Nicolas> tmp = readl(trng->base + TRNG_ODATA);
>  Nicolas> if (readl(trng->base + TRNG_ISR) & 1) {
>  Nicolas> 	*data = tmp;
>  Nicolas> 	return 4;
>  Nicolas> } else {
>  Nicolas>   	return 0;
>  Nicolas> }
> 
> No, that won't work as you then have another race. Data might not be
> ready when you read ODATA, but then become ready just in time for when
> you read ISR, so you end up using stale data.

Yes, sure.

> It all would have been easier if the ready bit would get cleared on
> reads from ODATA instead of/as well as from ISR, but that's
> unfortunately not the case.

I will talk about that idea to my friends designers and see if we can
improve things, in the future...

Thanks, bye,
-- 
Nicolas Ferre
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