Saner locking around deactivate_super()
Make sure that s_umount is acquired *before* we drop the final
active reference; we still have the fast path (atomic_dec_unless)
and we have gotten rid of the window between the moment when
s_active hits zero and s_umount is acquired. Which simplifies
the living hell out of grab_super() and inotify pin_to_kill()
stuff.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/notify/inotify/inotify.c b/fs/notify/inotify/inotify.c
index 441ef13..27b75eb 100644
--- a/fs/notify/inotify/inotify.c
+++ b/fs/notify/inotify/inotify.c
@@ -511,34 +511,8 @@
* done. Cleanup is just deactivate_super(). However, that leaves a messy
* case - what if we *are* racing with umount() and active references to
* superblock can't be acquired anymore? We can bump ->s_count, grab
- * ->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 (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
- * the damn thing still alive or we'd lost not one but two races at once,
- * the watch had been killed and a new one got created with the same ->wd
- * at the same address. That couldn't have happened in inotify_destroy(),
- * but inotify_rm_wd() could run into that. Still, "new one got created"
- * is not a problem - we have every right to kill it or leave it alone,
- * whatever's more convenient.
- *
- * So we can use idr_find(...) == watch && watch->inode->i_sb == sb as
- * "grab it and kill it" check. If it's been our original watch, we are
- * fine, if it's a newcomer - nevermind, just pretend that we'd won the
- * race and kill the fscker anyway; we are safe since we know that its
- * superblock won't be going away.
+ * ->s_umount, which will wait until the superblock is shut down and the
+ * watch in question is pining for fjords.
*
* And yes, this is far beyond mere "not very pretty"; so's the entire
* concept of inotify to start with.
@@ -552,14 +526,10 @@
* Called with ih->mutex held, drops it. Possible return values:
* 0 - nothing to do, it has died
* 1 - remove it, drop the reference and deactivate_super()
- * 2 - remove it, drop the reference and drop_super(); we tried hard to avoid
- * that variant, since it involved a lot of PITA, but that's the best that
- * could've been done.
*/
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;
if (atomic_inc_not_zero(&sb->s_active)) {
get_inotify_watch(watch);
@@ -571,36 +541,16 @@
spin_unlock(&sb_lock);
mutex_unlock(&ih->mutex); /* can't grab ->s_umount under it */
down_read(&sb->s_umount);
- if (likely(!sb->s_root)) {
- /* fs is already shut down; the watch is dead */
- drop_super(sb);
- return 0;
- }
- /* raced with the final deactivate_super() */
- mutex_lock(&ih->mutex);
- if (idr_find(&ih->idr, wd) != watch || watch->inode->i_sb != sb) {
- /* the watch is dead */
- mutex_unlock(&ih->mutex);
- drop_super(sb);
- return 0;
- }
- /* still alive or freed and reused with the same sb and wd; kill */
- get_inotify_watch(watch);
- mutex_unlock(&ih->mutex);
- return 2;
+ /* fs is already shut down; the watch is dead */
+ drop_super(sb);
+ return 0;
}
-static void unpin_and_kill(struct inotify_watch *watch, int how)
+static void unpin_and_kill(struct inotify_watch *watch)
{
struct super_block *sb = watch->inode->i_sb;
put_inotify_watch(watch);
- switch (how) {
- case 1:
- deactivate_super(sb);
- break;
- case 2:
- drop_super(sb);
- }
+ deactivate_super(sb);
}
/**
@@ -622,7 +572,6 @@
struct list_head *watches;
struct super_block *sb;
struct inode *inode;
- int how;
mutex_lock(&ih->mutex);
watches = &ih->watches;
@@ -632,8 +581,7 @@
}
watch = list_first_entry(watches, struct inotify_watch, h_list);
sb = watch->inode->i_sb;
- how = pin_to_kill(ih, watch);
- if (!how)
+ if (!pin_to_kill(ih, watch))
continue;
inode = watch->inode;
@@ -648,7 +596,7 @@
mutex_unlock(&ih->mutex);
mutex_unlock(&inode->inotify_mutex);
- unpin_and_kill(watch, how);
+ unpin_and_kill(watch);
}
/* free this handle: the put matching the get in inotify_init() */
@@ -851,7 +799,6 @@
struct inotify_watch *watch;
struct super_block *sb;
struct inode *inode;
- int how;
mutex_lock(&ih->mutex);
watch = idr_find(&ih->idr, wd);
@@ -860,8 +807,7 @@
return -EINVAL;
}
sb = watch->inode->i_sb;
- how = pin_to_kill(ih, watch);
- if (!how)
+ if (!pin_to_kill(ih, watch))
return 0;
inode = watch->inode;
@@ -875,7 +821,7 @@
mutex_unlock(&ih->mutex);
mutex_unlock(&inode->inotify_mutex);
- unpin_and_kill(watch, how);
+ unpin_and_kill(watch);
return 0;
}
diff --git a/fs/super.c b/fs/super.c
index bc734f8..157657b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -178,38 +178,15 @@
/**
- * deactivate_super - drop an active reference to superblock
- * @s: superblock to deactivate
- *
- * Drops an active reference to superblock, acquiring a temprory one if
- * there is no active references left. In that case we lock superblock,
- * tell fs driver to shut it down and drop the temporary reference we
- * had just acquired.
- */
-void deactivate_super(struct super_block *s)
-{
- struct file_system_type *fs = s->s_type;
- if (atomic_dec_and_test(&s->s_active)) {
- vfs_dq_off(s, 0);
- down_write(&s->s_umount);
- fs->kill_sb(s);
- put_filesystem(fs);
- put_super(s);
- }
-}
-
-EXPORT_SYMBOL(deactivate_super);
-
-/**
* deactivate_locked_super - drop an active reference to superblock
* @s: superblock to deactivate
*
- * Equivalent of up_write(&s->s_umount); deactivate_super(s);, except that
- * it does not unlock it until it's all over. As the result, it's safe to
- * use to dispose of new superblock on ->get_sb() failure exits - nobody
- * will see the sucker until it's all over. Equivalent using up_write +
- * deactivate_super is safe for that purpose only if superblock is either
- * safe to use or has NULL ->s_root when we unlock.
+ * Drops an active reference to superblock, converting it into a temprory
+ * one if there is no other active references left. In that case we
+ * tell fs driver to shut it down and drop the temporary reference we
+ * had just acquired.
+ *
+ * Caller holds exclusive lock on superblock; that lock is released.
*/
void deactivate_locked_super(struct super_block *s)
{
@@ -227,6 +204,24 @@
EXPORT_SYMBOL(deactivate_locked_super);
/**
+ * deactivate_super - drop an active reference to superblock
+ * @s: superblock to deactivate
+ *
+ * Variant of deactivate_locked_super(), except that superblock is *not*
+ * locked by caller. If we are going to drop the final active reference,
+ * lock will be acquired prior to that.
+ */
+void deactivate_super(struct super_block *s)
+{
+ if (!atomic_add_unless(&s->s_active, -1, 1)) {
+ down_write(&s->s_umount);
+ deactivate_locked_super(s);
+ }
+}
+
+EXPORT_SYMBOL(deactivate_super);
+
+/**
* grab_super - acquire an active reference
* @s: reference we are trying to make active
*
@@ -247,12 +242,10 @@
/* it's going away */
s->s_count++;
spin_unlock(&sb_lock);
- /* usually that'll be enough for it to die... */
+ /* wait for it to die */
down_write(&s->s_umount);
up_write(&s->s_umount);
put_super(s);
- /* ... but in case it wasn't, let's at least yield() */
- yield();
return 0;
}
