[PATCH v3 hmm 06/12] mm/hmm: Hold on to the mmget for the lifetime of the range

[Jason Gunthorpe]

On Wed, Jun 19, 2019 at 01:18:58AM -0700, Christoph Hellwig wrote:
> >  	mutex_lock(&hmm->lock);
> > -	list_del(&range->list);
> > +	list_del_init(&range->list);
> >  	mutex_unlock(&hmm->lock);
> 
> I don't see the point why this is a list_del_init - that just
> reinitializeѕ range->list, but doesn't change anything for the list
> head it was removed from.  (and if the list_del_init was intended
> a later patch in your branch reverts it to plain list_del..)

Just following the instructions:

/**
 * list_empty_careful - tests whether a list is empty and not being modified
 * @head: the list to test
 *
 * Description:
 * tests whether a list is empty _and_ checks that no other CPU might be
 * in the process of modifying either member (next or prev)
 *
 * NOTE: using list_empty_careful() without synchronization
 * can only be safe if the only activity that can happen
 * to the list entry is list_del_init(). Eg. it cannot be used
 * if another CPU could re-list_add() it.
 */

Agree it doesn't seem obvious why this is relevant when checking the
list head..

Maybe the comment is a bit misleading?

Jason