Messages in this thread | | | From | Miklos Szeredi <> | Date | Wed, 22 Aug 2018 21:58:15 +0200 | Subject | Re: [PATCH] ovl: set I_CREATING on inode being created |
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On Wed, Aug 22, 2018 at 4:53 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Wed, Aug 22, 2018 at 1:55 AM Miklos Szeredi <miklos@szeredi.hu> 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.
Thanks, Miklos
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