net: Add a mutex around devnet_rename_seq

On RT write_seqcount_begin() disables preemption and device_rename()
allocates memory with GFP_KERNEL and grabs later the sysfs_mutex
mutex. Serialize with a mutex and add use the non preemption disabling
__write_seqcount_begin().

To avoid writer starvation, let the reader grab the mutex and release
it when it detects a writer in progress. This keeps the normal case
(no reader on the fly) fast.

[ tglx: Instead of replacing the seqcount by a mutex, add the mutex ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 20 insertions, 14 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index ebe863c..da03d72 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -175,6 +175,7 @@ static unsigned int napi_gen_id;
 static DEFINE_HASHTABLE(napi_hash, 8);
 
 static seqcount_t devnet_rename_seq;
+static DEFINE_MUTEX(devnet_rename_mutex);
 
 static inline void dev_base_seq_inc(struct net *net)
 {
@@ -826,7 +827,8 @@ retry:
 	strcpy(name, dev->name);
 	rcu_read_unlock();
 	if (read_seqcount_retry(&devnet_rename_seq, seq)) {
-		cond_resched();
+		mutex_lock(&devnet_rename_mutex);
+		mutex_unlock(&devnet_rename_mutex);
 		goto retry;
 	}
 
@@ -1092,30 +1094,28 @@ int dev_change_name(struct net_device *dev, const char *newname)
 	if (dev->flags & IFF_UP)
 		return -EBUSY;
 
-	write_seqcount_begin(&devnet_rename_seq);
+	mutex_lock(&devnet_rename_mutex);
+	__write_seqcount_begin(&devnet_rename_seq);
 
-	if (strncmp(newname, dev->name, IFNAMSIZ) == 0) {
-		write_seqcount_end(&devnet_rename_seq);
-		return 0;
-	}
+	if (strncmp(newname, dev->name, IFNAMSIZ) == 0)
+		goto outunlock;
 
 	memcpy(oldname, dev->name, IFNAMSIZ);
 
 	err = dev_get_valid_name(net, dev, newname);
-	if (err < 0) {
-		write_seqcount_end(&devnet_rename_seq);
-		return err;
-	}
+	if (err < 0)
+		goto outunlock;
 
 rollback:
 	ret = device_rename(&dev->dev, dev->name);
 	if (ret) {
 		memcpy(dev->name, oldname, IFNAMSIZ);
-		write_seqcount_end(&devnet_rename_seq);
-		return ret;
+		err = ret;
+		goto outunlock;
 	}
 
-	write_seqcount_end(&devnet_rename_seq);
+	__write_seqcount_end(&devnet_rename_seq);
+	mutex_unlock(&devnet_rename_mutex);
 
 	write_lock_bh(&dev_base_lock);
 	hlist_del_rcu(&dev->name_hlist);
@@ -1134,7 +1134,8 @@ rollback:
 		/* err >= 0 after dev_alloc_name() or stores the first errno */
 		if (err >= 0) {
 			err = ret;
-			write_seqcount_begin(&devnet_rename_seq);
+			mutex_lock(&devnet_rename_mutex);
+			__write_seqcount_begin(&devnet_rename_seq);
 			memcpy(dev->name, oldname, IFNAMSIZ);
 			goto rollback;
 		} else {
@@ -1144,6 +1145,11 @@ rollback:
 	}
 
 	return err;
+
+outunlock:
+	__write_seqcount_end(&devnet_rename_seq);
+	mutex_unlock(&devnet_rename_mutex);
+	return err;
 }
 
 /**