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Date: Wed, 14 Apr 2010 08:41:19 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: David Miller <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] fasync: RCU locking
On Wed, Apr 14, 2010 at 09:42:41AM +0200, Eric Dumazet wrote:
> 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 ?
It looks like all the read-side critical sections are protected by
rcu_read_lock(), so call_rcu() should be OK. And it is OK to invoke
call_rcu() with irqs disabled. (Just don't try it in an NMI handler.)
Or am I missing some code path that tries to use disabling of irqs
instead of using rcu_read_lock()? That happens to work in the current
implementation, but...
Thanx, Paul
> 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@...il.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;
> }
>
>
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