Re: [PATCH 6/6] mm: add preempt points into __purge_vmap_area_lazy

From: Steven Rostedt
Date: Tue Oct 18 2016 - 17:02:33 EST

On Tue, 18 Oct 2016 16:56:48 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> Is releasing the lock within a llist_for_each_entry_safe() actually safe? Is
> vmap_area_lock the one to protect the valist?
> That is llist_for_each_entry_safe(va, n_va, valist, purg_list) does:
> for (va = llist_entry(valist, typeof(*va), purge_list);
> &va->purge_list != NULL &&
> n_va = llist_entry(va->, typeof(*n_va),
> purge_list, true);
> pos = n)
> Thus n_va is pointing to the next element to process when we release the
> lock. Is it possible for another task to get into this same path and process
> the item that n_va is pointing to? Then when the preempted task comes back,
> grabs the vmap_area_lock, and then continues the loop with what n_va has,
> could that cause problems? That is, the next iteration after releasing the
> lock does va = n_va. What happens if n_va no longer exits?
> I don't know this code that well, and perhaps vmap_area_lock is not protecting
> the list and this is all fine.

Bah, nevermind. I missed the:

valist = llist_del_all(&vmap_purge_list);

so yeah, all should be good.

Nothing to see here, move along please.

-- Steve