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    Subject[PATCH 3/3] slub: release a lock if freeing object with a lock is failed in __slab_free()
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    In some case of __slab_free(), we need a lock for manipulating partial list.
    If freeing object with a lock is failed, a lock doesn't needed anymore
    for some reasons.

    Case 1. prior is NULL, kmem_cache_debug(s) is true

    In this case, another free is occured before our free is succeed.
    When slab is full(prior is NULL), only possible operation is slab_free().
    So in this case, we guess another free is occured.
    It may make a slab frozen, so lock is not needed anymore.

    Case 2. inuse is NULL

    In this case, acquire_slab() is occured before out free is succeed.
    We have a last object for slab, so other operation for this slab is
    not possible except acquire_slab().
    Acquire_slab() makes a slab frozen, so lock is not needed anymore.

    Above two reason explain why we don't need a lock
    when freeing object with a lock is failed.

    So, when cmpxchg_double_slab() is failed, releasing a lock is more suitable.
    This may reduce lock contention.

    This also make logic somehow simple that 'was_frozen with a lock' case
    is never occured. Remove it.

    Signed-off-by: Joonsoo Kim <js1304@gmail.com>

    diff --git a/mm/slub.c b/mm/slub.c
    index 531d8ed..3e0b9db 100644
    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -2438,7 +2438,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
    void *prior;
    void **object = (void *)x;
    int was_frozen;
    - int inuse;
    struct page new;
    unsigned long counters;
    struct kmem_cache_node *n = NULL;
    @@ -2450,13 +2449,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
    return;

    do {
    + if (unlikely(n)) {
    + spin_unlock_irqrestore(&n->list_lock, flags);
    + n = NULL;
    + }
    prior = page->freelist;
    counters = page->counters;
    set_freepointer(s, object, prior);
    new.counters = counters;
    was_frozen = new.frozen;
    new.inuse--;
    - if ((!new.inuse || !prior) && !was_frozen && !n) {
    + if ((!new.inuse || !prior) && !was_frozen) {

    if (!kmem_cache_debug(s) && !prior)

    @@ -2481,7 +2484,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,

    }
    }
    - inuse = new.inuse;

    } while (!cmpxchg_double_slab(s, page,
    prior, counters,
    @@ -2507,25 +2509,17 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
    return;
    }

    + if (unlikely(!new.inuse && n->nr_partial > s->min_partial))
    + goto slab_empty;
    +
    /*
    - * was_frozen may have been set after we acquired the list_lock in
    - * an earlier loop. So we need to check it here again.
    + * Objects left in the slab. If it was not on the partial list before
    + * then add it.
    */
    - if (was_frozen)
    - stat(s, FREE_FROZEN);
    - else {
    - if (unlikely(!inuse && n->nr_partial > s->min_partial))
    - goto slab_empty;
    -
    - /*
    - * Objects left in the slab. If it was not on the partial list before
    - * then add it.
    - */
    - if (unlikely(!prior)) {
    - remove_full(s, page);
    - add_partial(n, page, DEACTIVATE_TO_TAIL);
    - stat(s, FREE_ADD_PARTIAL);
    - }
    + if (kmem_cache_debug(s) && unlikely(!prior)) {
    + remove_full(s, page);
    + add_partial(n, page, DEACTIVATE_TO_TAIL);
    + stat(s, FREE_ADD_PARTIAL);
    }
    spin_unlock_irqrestore(&n->list_lock, flags);
    return;
    --
    1.7.9.5


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