[PATCH net-next-2.6] fasync: RCU locking

[Eric Dumazet <eric.dumazet@gmail.com>]

Paul, could you please check this patch, I am not sure
of the IRQ safety thing...

Is call_rcu() the right method to use in this case ?

Thanks

[PATCH net-next-2.6] fasync: RCU locking

kill_fasync() uses a central rwlock, candidate for RCU conversion.

We can remove __kill_fasync() direct use in net, and rename it to
kill_fasync_rcu()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 fs/fcntl.c         |   36 +++++++++++++++++++++---------------
 include/linux/fs.h |   11 +++++------
 net/socket.c       |    4 ++--
 3 files changed, 28 insertions(+), 23 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 452d02f..33cb3ee 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -614,9 +614,15 @@ int send_sigurg(struct fown_struct *fown)
 	return ret;
 }
 
-static DEFINE_RWLOCK(fasync_lock);
+static DEFINE_SPINLOCK(fasync_lock);
 static struct kmem_cache *fasync_cache __read_mostly;
 
+static void fasync_free_rcu(struct rcu_head *head)
+{
+	kmem_cache_free(fasync_cache,
+			container_of(head, struct fasync_struct, fa_rcu));
+}
+
 /*
  * Remove a fasync entry. If successfully removed, return
  * positive and clear the FASYNC flag. If no entry exists,
@@ -634,17 +640,17 @@ static int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
 	int result = 0;
 
 	spin_lock(&filp->f_lock);
-	write_lock_irq(&fasync_lock);
+	spin_lock_irq(&fasync_lock);
 	for (fp = fapp; (fa = *fp) != NULL; fp = &fa->fa_next) {
 		if (fa->fa_file != filp)
 			continue;
 		*fp = fa->fa_next;
-		kmem_cache_free(fasync_cache, fa);
+		call_rcu(&fa->fa_rcu, fasync_free_rcu);
 		filp->f_flags &= ~FASYNC;
 		result = 1;
 		break;
 	}
-	write_unlock_irq(&fasync_lock);
+	spin_unlock_irq(&fasync_lock);
 	spin_unlock(&filp->f_lock);
 	return result;
 }
@@ -666,7 +672,7 @@ static int fasync_add_entry(int fd, struct file *filp, struct fasync_struct **fa
 		return -ENOMEM;
 
 	spin_lock(&filp->f_lock);
-	write_lock_irq(&fasync_lock);
+	spin_lock_irq(&fasync_lock);
 	for (fp = fapp; (fa = *fp) != NULL; fp = &fa->fa_next) {
 		if (fa->fa_file != filp)
 			continue;
@@ -679,12 +685,12 @@ static int fasync_add_entry(int fd, struct file *filp, struct fasync_struct **fa
 	new->fa_file = filp;
 	new->fa_fd = fd;
 	new->fa_next = *fapp;
-	*fapp = new;
+	rcu_assign_pointer(*fapp, new);
 	result = 1;
 	filp->f_flags |= FASYNC;
 
 out:
-	write_unlock_irq(&fasync_lock);
+	spin_unlock_irq(&fasync_lock);
 	spin_unlock(&filp->f_lock);
 	return result;
 }
@@ -704,7 +710,10 @@ int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fap
 
 EXPORT_SYMBOL(fasync_helper);
 
-void __kill_fasync(struct fasync_struct *fa, int sig, int band)
+/*
+ * rcu_read_lock() is held
+ */
+static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
 {
 	while (fa) {
 		struct fown_struct * fown;
@@ -719,22 +728,19 @@ void __kill_fasync(struct fasync_struct *fa, int sig, int band)
 		   mechanism. */
 		if (!(sig == SIGURG && fown->signum == 0))
 			send_sigio(fown, fa->fa_fd, band);
-		fa = fa->fa_next;
+		fa = rcu_dereference(fa->fa_next);
 	}
 }
 
-EXPORT_SYMBOL(__kill_fasync);
-
 void kill_fasync(struct fasync_struct **fp, int sig, int band)
 {
 	/* First a quick test without locking: usually
 	 * the list is empty.
 	 */
 	if (*fp) {
-		read_lock(&fasync_lock);
-		/* reread *fp after obtaining the lock */
-		__kill_fasync(*fp, sig, band);
-		read_unlock(&fasync_lock);
+		rcu_read_lock();
+		kill_fasync_rcu(rcu_dereference(*fp), sig, band);
+		rcu_read_unlock();
 	}
 }
 EXPORT_SYMBOL(kill_fasync);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 39d57bc..158b2cc 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1280,10 +1280,11 @@ static inline int lock_may_write(struct inode *inode, loff_t start,
 
 
 struct fasync_struct {
-	int	magic;
-	int	fa_fd;
-	struct	fasync_struct	*fa_next; /* singly linked list */
-	struct	file 		*fa_file;
+	int			magic;
+	int			fa_fd;
+	struct fasync_struct	*fa_next; /* singly linked list */
+	struct file		*fa_file;
+	struct rcu_head		fa_rcu;
 };
 
 #define FASYNC_MAGIC 0x4601
@@ -1292,8 +1293,6 @@ struct fasync_struct {
 extern int fasync_helper(int, struct file *, int, struct fasync_struct **);
 /* can be called from interrupts */
 extern void kill_fasync(struct fasync_struct **, int, int);
-/* only for net: no internal synchronization */
-extern void __kill_fasync(struct fasync_struct *, int, int);
 
 extern int __f_setown(struct file *filp, struct pid *, enum pid_type, int force);
 extern int f_setown(struct file *filp, unsigned long arg, int force);
diff --git a/net/socket.c b/net/socket.c
index 35bc198..846739c 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1159,10 +1159,10 @@ int sock_wake_async(struct socket *sock, int how, int band)
 		/* fall through */
 	case SOCK_WAKE_IO:
 call_kill:
-		__kill_fasync(sock->fasync_list, SIGIO, band);
+		kill_fasync(sock->fasync_list, SIGIO, band);
 		break;
 	case SOCK_WAKE_URG:
-		__kill_fasync(sock->fasync_list, SIGURG, band);
+		kill_fasync(sock->fasync_list, SIGURG, band);
 	}
 	return 0;
 }