Re: åå: [PATCH] mm/slab.c: add node spinlock protect in __cache_free_alien

[David Rientjes]

On Wed, 29 Jul 2020, Zhang, Qiang wrote:

> > From: Zhang Qiang <qiang.zhang@xxxxxxxxxxxxx>
> >
> > We should add node spinlock protect "n->alien" which may be
> > assigned to NULL in cpuup_canceled func. cause address access
> > exception.
> >
> 
> >Hi, do you have an example NULL pointer dereference where you have hit
> >this?
> 

If you have a NULL pointer dereference or a GPF that occurred because of 
this, it would be helpful to provide as rationale.

> >This rather looks like something to fix up in cpuup_canceled() since it's
> >currently manipulating the alien cache for the canceled cpu's node.
> 
> yes , it is fix up in cpuup_canceled  it's
> currently manipulating the alien cache for the canceled cpu's node which  may be the same as the node being operated on in the __cache_free_alien func.
> 
> void cpuup_canceled
> {
>         n = get_node(cachep, node);
>         spin_lock_irq(&n->list_lock);
>         ...
>         n->alien = NULL;
>         spin_unlock_irq(&n->list_lock);
>          ....
> }
> 

Right, so the idea is that this should be fixed in cpuup_canceled() 
instead -- why would we invaliate the entire node's alien cache because a 
single cpu failed to come online?