linux-kernel - Re: [PATCH 3/3] slub: release a lock if freeing object with a lock is failed in __slab

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Date:	Fri, 6 Jul 2012 09:34:01 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	JoonSoo Kim <js1304@...il.com>
cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 3/3] slub: release a lock if freeing object with a lock
 is failed in __slab_free()

On Fri, 6 Jul 2012, JoonSoo Kim wrote:

> For example,
> When we try to free object A at cpu 1, another process try to free
> object B at cpu 2 at the same time.
> object A, B is in same slab, and this slab is in full list.
>
> CPU 1                           CPU 2
> prior = page->freelist;    prior = page->freelist
> ....                                  ...
> new.inuse--;                   new.inuse--;
> taking lock                      try to take the lock, but failed, so
> spinning...
> free success                   spinning...
> add_partial
> release lock                    taking lock
>                                        fail cmpxchg_double_slab
>                                        retry
>                                        currently, we don't need lock
>
> At CPU2, we don't need lock anymore, because this slab already in partial list.

For that scenario we could also simply do a trylock there and redo
the loop if we fail. But still what guarantees that another process will
not modify the page struct between fetching the data and a successful
trylock?
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