linux-kernel - Re: [PATCH v2 1/3] virtio-rng cleanup: move some code out of mutex protection

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Date:	Mon, 15 Sep 2014 18:13:20 +0200
From:	Michael Büsch <m@...s.ch>
To:	Amos Kong <akong@...hat.com>
Cc:	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	herbert@...dor.apana.org.au, mpm@...enic.com,
	rusty@...tcorp.com.au, amit.shah@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] virtio-rng cleanup: move some code out of mutex
 protection

On Tue, 16 Sep 2014 00:02:27 +0800
Amos Kong <akong@...hat.com> wrote:

> It doesn't save too much cpu time as expected, just a cleanup.
> 
> Signed-off-by: Amos Kong <akong@...hat.com>
> ---
>  drivers/char/hw_random/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index aa30a25..c591d7e 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -270,8 +270,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
>  		return -ERESTARTSYS;
>  	if (current_rng)
>  		name = current_rng->name;
> -	ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
>  	mutex_unlock(&rng_mutex);
> +	ret = snprintf(buf, PAGE_SIZE, "%s\n", name);

I'm not sure this is safe.
Name is just a pointer.
What if the hwrng gets unregistered after unlock and just before the snprintf?

>  	return ret;
>  }
> @@ -284,19 +284,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
>  	ssize_t ret = 0;
>  	struct hwrng *rng;
>  
> +	buf[0] = '\0';
>  	err = mutex_lock_interruptible(&rng_mutex);
>  	if (err)
>  		return -ERESTARTSYS;
> -	buf[0] = '\0';
>  	list_for_each_entry(rng, &rng_list, list) {
>  		strncat(buf, rng->name, PAGE_SIZE - ret - 1);
>  		ret += strlen(rng->name);
>  		strncat(buf, " ", PAGE_SIZE - ret - 1);
>  		ret++;
>  	}
> +	mutex_unlock(&rng_mutex);
>  	strncat(buf, "\n", PAGE_SIZE - ret - 1);
>  	ret++;
> -	mutex_unlock(&rng_mutex);
>  
>  	return ret;
>  }

This looks ok.

-- 
Michael

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