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Subject[PATCH 3/4] Move FASYNC bit handling to f_op->fasync()
Date
Removing the BKL from FASYNC handling ran into the challenge of keeping the
setting of the FASYNC bit in filp->f_flags atomic with regard to calls to
the underlying fasync() function. Andi Kleen suggested moving the handling
of that bit into fasync(); this patch does exactly that.

As it happens, every fasync() implementation in the kernel with one
exception calls fasync_helper(). So, if we make fasync_helper() set the
FASYNC bit, we can avoid making any changes to the other fasync()
functions - as long as those functions, themselves, have proper locking.
Most fasync() implementations do nothing but call fasync_helper() - which
has its own lock - so they are easily verified as correct. The BKL had
already been pushed down into the few exceptions.

The networking code has its own version of fasync_helper(), so that code
has been augmented with explicit FASYNC bit handling.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
---
fs/fcntl.c | 21 ++++++++-------------
fs/ioctl.c | 11 +----------
net/socket.c | 2 ++
3 files changed, 11 insertions(+), 23 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 756119e..7b641d9 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -185,19 +185,14 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
return error;

/*
- * We still need a lock here for now to keep multiple FASYNC calls
- * from racing with each other.
+ * ->fasync() is responsible for setting the FASYNC bit.
*/
- lock_kernel();
- if ((arg ^ filp->f_flags) & FASYNC) {
- if (filp->f_op && filp->f_op->fasync) {
- error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);
- if (error < 0)
- goto out;
- }
- change_bit(NR_FASYNC, &filp->f_flags);
+ if (((arg ^ filp->f_flags) & FASYNC) && filp->f_op &&
+ filp->f_op->fasync) {
+ error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);
+ if (error < 0)
+ goto out;
}
-
/*
* Now that we use bitops we have to tweak each bit individually.
*/
@@ -207,7 +202,6 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
tweak_flags_bit(NR_O_DIRECT, arg & O_DIRECT, &filp->f_flags);
tweak_flags_bit(NR_O_NOATIME, arg & O_NOATIME, &filp->f_flags);
out:
- unlock_kernel();
return error;
}

@@ -532,7 +526,7 @@ static DEFINE_RWLOCK(fasync_lock);
static struct kmem_cache *fasync_cache __read_mostly;

/*
- * fasync_helper() is used by some character device drivers (mainly mice)
+ * fasync_helper() is used by almost all character device drivers
* to set up the fasync queue. It returns negative on error, 0 if it did
* no changes and positive if it added/deleted the entry.
*/
@@ -548,6 +542,7 @@ int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fap
return -ENOMEM;
}
write_lock_irq(&fasync_lock);
+ tweak_flags_bit(NR_FASYNC, on, &filp->f_flags);
for (fp = fapp; (fa = *fp) != NULL; fp = &fa->fa_next) {
if (fa->fa_file == filp) {
if(on) {
diff --git a/fs/ioctl.c b/fs/ioctl.c
index fc0db36..bc32cb2 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -427,17 +427,11 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
/* Did FASYNC state change ? */
if ((flag ^ filp->f_flags) & FASYNC) {
if (filp->f_op && filp->f_op->fasync)
+ /* fasync() adjusts filp->f_flags */
error = filp->f_op->fasync(fd, filp, on);
else
error = -ENOTTY;
}
- if (error)
- return error;
-
- if (on)
- set_bit(NR_FASYNC, &filp->f_flags);
- else
- clear_bit(NR_FASYNC, &filp->f_flags);
return error;
}

@@ -505,10 +499,7 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
break;

case FIOASYNC:
- /* BKL needed to avoid races tweaking f_flags */
- lock_kernel();
error = ioctl_fioasync(fd, filp, argp);
- unlock_kernel();
break;

case FIOQSIZE:
diff --git a/net/socket.c b/net/socket.c
index 35dd737..2b1e380 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1037,6 +1037,7 @@ static int sock_fasync(int fd, struct file *filp, int on)
break;

if (on) {
+ set_bit(NR_FASYNC, &filp->f_flags);
if (fa != NULL) {
write_lock_bh(&sk->sk_callback_lock);
fa->fa_fd = fd;
@@ -1053,6 +1054,7 @@ static int sock_fasync(int fd, struct file *filp, int on)
sock->fasync_list = fna;
write_unlock_bh(&sk->sk_callback_lock);
} else {
+ clear_bit(NR_FASYNC, &filp->f_flags);
if (fa != NULL) {
write_lock_bh(&sk->sk_callback_lock);
*prev = fa->fa_next;
--
1.6.1.1


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