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

Amos Kong
2014-09-16T00:32:18Z

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On Mon, Sep 15, 2014 at 06:13:20PM +0200, Michael Büsch wrote:
> On Tue, 16 Sep 2014 00:02:27 +0800
> Amos Kong <akong@redhat.com> wrote:
>
> > It doesn't save too much cpu time as expected, just a cleanup.
> >
> > Signed-off-by: Amos Kong <akong@redhat.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?

Oh, it points to protected current_rng->name, I will drop this
cleanup. Thanks.

> > 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

--
Amos.

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