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Date: Wed, 24 Mar 2010 16:12:23 +1100
From: NeilBrown <neilb@...e.de>
To: Al Viro <viro@...IV.linux.org.uk>
Cc: Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] fs/super: remove S_BIAS
The fact that S_BIAS is very large is only used four times,
all(*) with:
if (s->s_count > S_BIAS) {
atomic_inc(&s->s_active);
The statement "s->s_count > S_BIAS" is exactly equivalent
to "atomic_read(&s->s_active) != 0", as the bias is subtracted
as soon as s_active becomes zero.
So the above test can more simply become:
if (atomic_inc_not_zero(&s->s_active)) {
With this in place, S_BIAS does not need to be large, and a value of
'1' will suit. This simplifies code in a number of places and
removes the need to take a spinlock in several cases.
(*) full disclosure: in two cases (inotify) it is
if (s->s_count >= S_BIAS {
however the logic still holds - that can only be true if
s_active is not zero.
Signed-off-by: NeilBrown <neilb@...e.de>
---
fs/notify/inotify/inotify.c | 33 ++++++++++++++-------------------
fs/super.c | 16 +++++-----------
include/linux/fs.h | 1 -
3 files changed, 19 insertions(+), 31 deletions(-)
diff --git a/fs/notify/inotify/inotify.c b/fs/notify/inotify/inotify.c
index 40b1cf9..44256e5 100644
--- a/fs/notify/inotify/inotify.c
+++ b/fs/notify/inotify/inotify.c
@@ -110,14 +110,10 @@ EXPORT_SYMBOL_GPL(get_inotify_watch);
int pin_inotify_watch(struct inotify_watch *watch)
{
struct super_block *sb = watch->inode->i_sb;
- spin_lock(&sb_lock);
- if (sb->s_count >= S_BIAS) {
- atomic_inc(&sb->s_active);
- spin_unlock(&sb_lock);
+ if (atomic_inc_not_zero(&sb->s_active)) {
atomic_inc(&watch->count);
return 1;
}
- spin_unlock(&sb_lock);
return 0;
}
@@ -518,16 +514,17 @@ EXPORT_SYMBOL_GPL(inotify_init_watch);
* ->s_umount, which will almost certainly wait until the superblock is shut
* down and the watch in question is pining for fjords. That's fine, but
* there is a problem - we might have hit the window between ->s_active
- * getting to 0 / ->s_count - below S_BIAS (i.e. the moment when superblock
- * is past the point of no return and is heading for shutdown) and the
- * moment when deactivate_super() acquires ->s_umount. We could just do
- * drop_super() yield() and retry, but that's rather antisocial and this
- * stuff is luser-triggerable. OTOH, having grabbed ->s_umount and having
- * found that we'd got there first (i.e. that ->s_root is non-NULL) we know
- * that we won't race with inotify_umount_inodes(). So we could grab a
- * reference to watch and do the rest as above, just with drop_super() instead
- * of deactivate_super(), right? Wrong. We had to drop ih->mutex before we
- * could grab ->s_umount. So the watch could've been gone already.
+ * getting to 0 (i.e. the moment when superblock is past the point of no
+ * return and is heading for shutdown) and the moment when
+ * deactivate_super() acquires ->s_umount. We could just do drop_super()
+ * yield() and retry, but that's rather antisocial and this stuff is
+ * luser-triggerable. OTOH, having grabbed ->s_umount and having found
+ * that we'd got there first (i.e. that ->s_root is non-NULL) we know that
+ * we won't race with inotify_umount_inodes(). So we could grab a
+ * reference to watch and do the rest as above, just with drop_super()
+ * instead of deactivate_super(), right? Wrong. We had to drop ih->mutex
+ * before we could grab ->s_umount. So the watch could've been gone
+ * already.
*
* That still can be dealt with - we need to save watch->wd, do idr_find()
* and compare its result with our pointer. If they match, we either have
@@ -565,14 +562,12 @@ static int pin_to_kill(struct inotify_handle *ih, struct inotify_watch *watch)
struct super_block *sb = watch->inode->i_sb;
s32 wd = watch->wd;
- spin_lock(&sb_lock);
- if (sb->s_count >= S_BIAS) {
- atomic_inc(&sb->s_active);
- spin_unlock(&sb_lock);
+ if (atomic_inc_not_zero(&sb->s_active)) {
get_inotify_watch(watch);
mutex_unlock(&ih->mutex);
return 1; /* the best outcome */
}
+ spin_lock(&sb_lock);
sb->s_count++;
spin_unlock(&sb_lock);
mutex_unlock(&ih->mutex); /* can't grab ->s_umount under it */
diff --git a/fs/super.c b/fs/super.c
index 34c8391..9806711 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -92,7 +92,7 @@ static struct super_block *alloc_super(struct file_system_type *type)
* subclass.
*/
down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
- s->s_count = S_BIAS;
+ s->s_count = 1;
atomic_set(&s->s_active, 1);
mutex_init(&s->s_vfs_rename_mutex);
mutex_init(&s->s_dquot.dqio_mutex);
@@ -188,9 +188,7 @@ void put_super(struct super_block *sb)
void deactivate_super(struct super_block *s)
{
struct file_system_type *fs = s->s_type;
- if (atomic_dec_and_lock(&s->s_active, &sb_lock)) {
- s->s_count -= S_BIAS-1;
- spin_unlock(&sb_lock);
+ if (atomic_dec_and_test(&s->s_active)) {
vfs_dq_off(s, 0);
down_write(&s->s_umount);
fs->kill_sb(s);
@@ -215,9 +213,7 @@ EXPORT_SYMBOL(deactivate_super);
void deactivate_locked_super(struct super_block *s)
{
struct file_system_type *fs = s->s_type;
- if (atomic_dec_and_lock(&s->s_active, &sb_lock)) {
- s->s_count -= S_BIAS-1;
- spin_unlock(&sb_lock);
+ if (atomic_dec_and_test(&s->s_active)) {
vfs_dq_off(s, 0);
fs->kill_sb(s);
put_filesystem(fs);
@@ -247,8 +243,7 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
down_write(&s->s_umount);
if (s->s_root) {
spin_lock(&sb_lock);
- if (s->s_count > S_BIAS) {
- atomic_inc(&s->s_active);
+ if (atomic_inc_not_zero(&s->s_active)) {
s->s_count--;
spin_unlock(&sb_lock);
return 1;
@@ -492,8 +487,7 @@ restart:
down_write(&sb->s_umount);
if (sb->s_root) {
spin_lock(&sb_lock);
- if (sb->s_count > S_BIAS) {
- atomic_inc(&sb->s_active);
+ if (atomic_inc_not_zero(&sb->s_active)) {
sb->s_count--;
spin_unlock(&sb_lock);
return sb;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 10b8ded..0e31ba8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1315,7 +1315,6 @@ extern struct list_head super_blocks;
extern spinlock_t sb_lock;
#define sb_entry(list) list_entry((list), struct super_block, s_list)
-#define S_BIAS (1<<30)
struct super_block {
struct list_head s_list; /* Keep this first */
dev_t s_dev; /* search index; _not_ kdev_t */
--
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