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

On Mon, Sep 15, 2014 at 06:13:20PM +0200, Michael Büsch wrote:

On Tue, 16 Sep 2014 00:02:27 +0800
Amos Kong [off-list ref] wrote:

quoted

It doesn't save too much cpu time as expected, just a cleanup.

Signed-off-by: Amos Kong <redacted>
---
 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.
 

quoted

 	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.