Re: [PATCH v2 (resend)] media: imon: reorganize serialization

Tetsuo Handa
2022-05-02T10:27:41Z

On 2022/05/02 18:39, Sean Young wrote:
> So this part of the patch address the issue of driver_lock being
> unnecessary. This should be in its own patch, so I am going to merge:
>
> https://patchwork.linuxtv.org/project/linux-media/patch/349f3e34-41ed-f832-3b22-ae10c50e3868@I-love.SAKURA.ne.jp/

driver_lock is not unnecessary, unless combined with kfree_rcu() approach.

>> + /*
>> + * We need to wait for RCU grace period in order to allow
>> + * display_open() to safely check ->disconnected and increment ->users.
>> + */
>> + struct rcu_head rcu;
>
> Is it possible to modify the users/disconnected fields while holding the
> lock mutex in imon_context? This would make it unnecessary to use rcu and
> simplify the code.

I don't think it is possible, and I don't think it simplifies the code.

Unless we revive global driver_lock lock (untested delta diff is shown below),
nothing prevents from calling kfree(ictx) before holding ictx->lock or
checking ->disconnected or incrementing ->users.

As a whole, I think kfree_rcu() is simpler while removing global lock.

diff --git a/drivers/media/rc/imon.c b/drivers/media/rc/imon.c
index 9a4f24e294bc..469a2f869572 100644
--- a/drivers/media/rc/imon.c
+++ b/drivers/media/rc/imon.c
@@ -166,11 +166,6 @@ struct imon_context {
* imon_disconnect() was already called.
*/
bool disconnected;
- /*
- * We need to wait for RCU grace period in order to allow
- * display_open() to safely check ->disconnected and increment ->users.
- */
- struct rcu_head rcu;
};

#define TOUCH_TIMEOUT (HZ/30)
@@ -457,6 +452,9 @@ static struct usb_driver imon_driver = {
.id_table = imon_usb_id_table,
};

+/* to prevent races between open() and kfree() */
+static DEFINE_MUTEX(driver_lock);
+
/* Module bookkeeping bits */
MODULE_AUTHOR(MOD_AUTHOR);
MODULE_DESCRIPTION(MOD_DESC);
@@ -516,6 +514,8 @@ static int display_open(struct inode *inode, struct file *file)
int subminor;
int retval = 0;

+ mutex_lock(&driver_lock);
+
subminor = iminor(inode);
interface = usb_find_interface(&imon_driver, subminor);
if (!interface) {
@@ -524,15 +524,13 @@ static int display_open(struct inode *inode, struct file *file)
goto exit;
}

- rcu_read_lock();
ictx = usb_get_intfdata(interface);
if (!ictx || ictx->disconnected || !refcount_inc_not_zero(&ictx->users)) {
- rcu_read_unlock();
pr_err("no context found for minor %d\n", subminor);
retval = -ENODEV;
goto exit;
}
- rcu_read_unlock();
+ mutex_unlock(&driver_lock);

mutex_lock(&ictx->lock);

@@ -550,10 +548,15 @@ static int display_open(struct inode *inode, struct file *file)

mutex_unlock(&ictx->lock);

- if (retval && refcount_dec_and_test(&ictx->users))
- free_imon_context(ictx);
-
+ if (retval) {
+ mutex_unlock(&driver_lock);
+ if (refcount_dec_and_test(&ictx->users))
+ free_imon_context(ictx);
+ mutex_unlock(&driver_lock);
+ }
+ return retval;
exit:
+ mutex_unlock(&driver_lock);
return retval;
}

@@ -2497,6 +2500,8 @@ static void imon_disconnect(struct usb_interface *interface)
struct device *dev;
int ifnum;

+ mutex_lock(&driver_lock);
+
ictx = usb_get_intfdata(interface);
ictx->disconnected = true;
dev = ictx->dev;
@@ -2542,6 +2547,8 @@ static void imon_disconnect(struct usb_interface *interface)
if (refcount_dec_and_test(&ictx->users))
free_imon_context(ictx);

+ mutex_unlock(&driver_lock);
+
dev_dbg(dev, "%s: iMON device (intf%d) disconnected\n",
__func__, ifnum);
}