Re: [PATCH] ovl: set I_CREATING on inode being created

From: Miklos Szeredi
Date: Wed Aug 22 2018 - 15:58:23 EST

On Wed, Aug 22, 2018 at 4:53 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Aug 22, 2018 at 1:55 AM Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> + spin_lock(&inode->i_lock);
>> + inode->i_state |= I_CREATING;
>> + spin_unlock(&inode->i_lock);
>> +
> Why is that spinlock protection there?
> Isn't this a new inode that cannot possibly be reached any other way yet?

new_inode() puts it on sb->s_inodes list, so it *is* reachable.
Following operate on s_inodes:

- evict_inodes() called from generic_shutdown_super(): a) we shouldn't
get here while in creation, b) it's careful to not touch inodes with
non-zero refcount

- invalidate_inodes(), called from block devices, so it doesn't apply
to overlayfs, also it skips inodes with non-zero refcount

- iterate_bdevs(), operates on blockdev_superblock

- fsnotify_unmount_inodes() called from generic_shutdown_super(): we
shouldn't get here while in creation,

- add_dquot_ref(), remove_dquot_ref(): not quite sure what these do,
but quotas are not (yet) supported on overlayfs

So looks like we are safe without a spinlock.

And there's another, more fundamental reason: if anything starts
messing with i_state of an inode that is not yet even had its state
changed to I_NEW, then lots of filesystems are in trouble.

> NOTE! This is a question. Maybe there is something I missed, and there
> *are* other ways to reach that inode. But if that's true, isn't it
> already too late to set I_CREATING?

No, it's not too late, I_CREATING can be set anytime up to
inode_insert5(), which is the first one to actually look at that flag.

> So I'd like some clarification on this point before applying it. It's
> possible that the spinlock is required, I just want to understand why.

I added the spinlock, because it's cheap (new_inode() already pulls it
into L1 cache) and because it's much harder to prove that lockless one
is correct than just adding that locking.