netdev - Re: [Patch net] tap: convert a mutex to a spinlock

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Date:   Thu, 6 Jul 2017 09:14:01 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     Sainath Grandhi <sainath.grandhi@...el.com>
Subject: Re: [Patch net] tap: convert a mutex to a spinlock

On 07/05/2017 10:50 PM, Cong Wang wrote:
> We are not allowed to block on the RCU reader side, so can't
> just hold the mutex as before. As a quick fix, convert it to
> a spinlock.
> 
> Fixes: d9f1f61c0801 ("tap: Extending tap device create/destroy APIs")
> Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
> Cc: Sainath Grandhi <sainath.grandhi@...el.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

Tested-by: Christian Borntraeger <borntraeger@...ibm.com>


> ---
>  drivers/net/tap.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index 4d4173d..d88ae3c 100644
> --- a/drivers/net/tap.c
> +++ b/drivers/net/tap.c
> @@ -106,7 +106,7 @@ struct major_info {
>  	struct rcu_head rcu;
>  	dev_t major;
>  	struct idr minor_idr;
> -	struct mutex minor_lock;
> +	spinlock_t minor_lock;
>  	const char *device_name;
>  	struct list_head next;
>  };
> @@ -416,15 +416,15 @@ int tap_get_minor(dev_t major, struct tap_dev *tap)
>  		goto unlock;
>  	}
> 
> -	mutex_lock(&tap_major->minor_lock);
> -	retval = idr_alloc(&tap_major->minor_idr, tap, 1, TAP_NUM_DEVS, GFP_KERNEL);
> +	spin_lock(&tap_major->minor_lock);
> +	retval = idr_alloc(&tap_major->minor_idr, tap, 1, TAP_NUM_DEVS, GFP_ATOMIC);
>  	if (retval >= 0) {
>  		tap->minor = retval;
>  	} else if (retval == -ENOSPC) {
>  		netdev_err(tap->dev, "Too many tap devices\n");
>  		retval = -EINVAL;
>  	}
> -	mutex_unlock(&tap_major->minor_lock);
> +	spin_unlock(&tap_major->minor_lock);
> 
>  unlock:
>  	rcu_read_unlock();
> @@ -442,12 +442,12 @@ void tap_free_minor(dev_t major, struct tap_dev *tap)
>  		goto unlock;
>  	}
> 
> -	mutex_lock(&tap_major->minor_lock);
> +	spin_lock(&tap_major->minor_lock);
>  	if (tap->minor) {
>  		idr_remove(&tap_major->minor_idr, tap->minor);
>  		tap->minor = 0;
>  	}
> -	mutex_unlock(&tap_major->minor_lock);
> +	spin_unlock(&tap_major->minor_lock);
> 
>  unlock:
>  	rcu_read_unlock();
> @@ -467,13 +467,13 @@ static struct tap_dev *dev_get_by_tap_file(int major, int minor)
>  		goto unlock;
>  	}
> 
> -	mutex_lock(&tap_major->minor_lock);
> +	spin_lock(&tap_major->minor_lock);
>  	tap = idr_find(&tap_major->minor_idr, minor);
>  	if (tap) {
>  		dev = tap->dev;
>  		dev_hold(dev);
>  	}
> -	mutex_unlock(&tap_major->minor_lock);
> +	spin_unlock(&tap_major->minor_lock);
> 
>  unlock:
>  	rcu_read_unlock();
> @@ -1227,7 +1227,7 @@ static int tap_list_add(dev_t major, const char *device_name)
>  	tap_major->major = MAJOR(major);
> 
>  	idr_init(&tap_major->minor_idr);
> -	mutex_init(&tap_major->minor_lock);
> +	spin_lock_init(&tap_major->minor_lock);
> 
>  	tap_major->device_name = device_name;
>