Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()

[Al Viro]

On Fri, Feb 23, 2018 at 03:58:14AM +0000, Al Viro wrote:
> On Fri, Feb 23, 2018 at 12:50:25AM +0100, John Ogness wrote:
> 
> > Avoid the trylock loop by using dentry_kill(). When killing dentries
> > from the dispose list, it is very similar to killing a dentry in
> > dput(). The difference is that dput() expects to be the last user of
> > the dentry (refcount=1) and will deref whereas shrink_dentry_list()
> > expects there to be no user (refcount=0). In order to handle both
> > situations with the same code, move the deref code from dentry_kill()
> > into a new wrapper function dentry_put_kill(), which can be used
> > by previous dentry_kill() users. Then shrink_dentry_list() can use
> > the dentry_kill() to cleanup the dispose list.
> > 
> > This also has the benefit that the locking order is now the same.
> > First the inode is locked, then the parent.
> 
> Current code moves the sucker to the end of list in that case; I'm not
> at all sure that what you are doing will improve the situation at all...
> 
> You *still* have a trylock loop there - only it keeps banging at the
> same dentry instead of going through the rest first...

Actually, it's even worse - _here_ you are dealing with something that
really can change inode under you.  This is one and only case where
we are kicking out a zero-refcount dentry without having already held
->i_lock.  At the very least, it's bloody different from regular
dentry_kill().  In this case, dentry itself is protected from freeing
by being on the shrink list - that's what makes __dentry_kill() to
leave the sucker allocated.   We are not holding references, it is
hashed and anybody could come, pick it, d_delete() it, etc.