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Saner locking around deactivate_super()

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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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57
fs/super.c
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@@ -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;
}