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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>
This commit is contained in:
Al Viro 2010-03-22 15:22:31 -04:00
parent b20bd1a5e7
commit 1712ac8fda
2 changed files with 36 additions and 97 deletions
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76
fs/notify/inotify/inotify.c
View File

@ -511,34 +511,8 @@ EXPORT_SYMBOL_GPL(inotify_init_watch);
* 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 @@ EXPORT_SYMBOL_GPL(inotify_init_watch);
* 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 @@ static int pin_to_kill(struct inotify_handle *ih, struct inotify_watch *watch)
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 @@ void inotify_destroy(struct inotify_handle *ih)
struct list_head *watches;
struct super_block *sb;
struct inode *inode;
int how;
mutex_lock(&ih->mutex);
watches = &ih->watches;
@ -632,8 +581,7 @@ void inotify_destroy(struct inotify_handle *ih)
}
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 @@ void inotify_destroy(struct inotify_handle *ih)
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 @@ int inotify_rm_wd(struct inotify_handle *ih, u32 wd)
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 @@ int inotify_rm_wd(struct inotify_handle *ih, u32 wd)
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 @@ int inotify_rm_wd(struct inotify_handle *ih, u32 wd)
mutex_unlock(&ih->mutex);
mutex_unlock(&inode->inotify_mutex);
unpin_and_kill(watch, how);
unpin_and_kill(watch);
return 0;
}

57
fs/super.c
View File

@ -177,39 +177,16 @@ void put_super(struct super_block *sb)
}
/**
* 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)
{
@ -226,6 +203,24 @@ void deactivate_locked_super(struct super_block *s)
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 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
/* 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;
}