linux-kernel - Re: [PATCH 6/6] mm: add preempt points into __purge_vmap_area

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Date:   Tue, 18 Oct 2016 17:01:47 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Christoph Hellwig <hch@....de>
Cc:     akpm@...ux-foundation.org, joelaf@...gle.com, jszhang@...vell.com,
        chris@...is-wilson.co.uk, joaodias@...gle.com, linux-mm@...ck.org,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] mm: add preempt points into __purge_vmap_area_lazy

On Tue, 18 Oct 2016 16:56:48 -0400
Steven Rostedt <rostedt@...dmis.org> 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->purge_list.next, 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