Re: [PATCH 3/3] slub: release a lock if freeing object with a lockis failed in __slab_free()

From: Christoph Lameter
Date: Fri Jul 06 2012 - 10:34:15 EST

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
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