Re: [PATCH 3/4] fs/dcache: Avoid the try_lock loop in d_delete()

[Peter Zijlstra]

On Fri, Feb 16, 2018 at 04:09:32PM +0100, John Ogness wrote:
> 
>    inode = dentry->d_inode;
>    rcu_read_lock();         <- Protects d_inode from being freed,
>                                i.e. dentry->d_inode is a valid pointer
>                                even after dentry->d_lock is dropped
>    unlock(dentry->d_lock);
>    lock(inode->i_lock);
>    lock(dentry->d_lock);
>    rcu_read_unlock();

So that is entirely tricky, typically we have to have a lookup _after_
rcu_read_lock().

Here, we rely on not being able to call dentry_free() while we hold
d_lock, which ensure dentry must be valid in the freshly started
rcu-section.

And I suppose that same ensures dentry->d_ionde stays alive. But this
needs a comment at least.