From: Linus Torvalds Date: Tue, 19 Feb 2008 17:08:49 +0000 (-0800) Subject: Revert "SLUB: Alternate fast paths using cmpxchg_local" X-Git-Tag: v2.6.25-rc3~120 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=00e962c5408b9f2d0bebd2308673fe982cb9a5fe;p=linux-2.6 Revert "SLUB: Alternate fast paths using cmpxchg_local" This reverts commit 1f84260c8ce3b1ce26d4c1d6dedc2f33a3a29c0c, which is suspected to be the reason for some very occasional and hard-to-trigger crashes that usually look related to memory allocation (mostly reported in networking, but since that's generally the most common source of shortlived allocations - and allocations in interrupt contexts - that in itself is not a big clue). See for example http://bugzilla.kernel.org/show_bug.cgi?id=9973 http://lkml.org/lkml/2008/2/19/278 etc. One promising suspicion for what the root cause of bug is (which also explains why it's so hard to trigger in practice) came from Eric Dumazet: "I wonder how SLUB_FASTPATH is supposed to work, since it is affected by a classical ABA problem of lockless algo. cmpxchg_local(&c->freelist, object, object[c->offset]) can succeed, while an interrupt came (on this cpu), and several allocations were done, and one free was performed at the end of this interruption, so 'object' was recycled. c->freelist can then contain the previous value (object), but object[c->offset] was changed by IRQ. We then put back in freelist an already allocated object." but another reason for the revert is simply that everybody agrees that this code was the main suspect just by virtue of the pattern of oopses. Cc: Torsten Kaiser Cc: Christoph Lameter Cc: Mathieu Desnoyers Cc: Pekka Enberg Cc: Ingo Molnar Cc: Eric Dumazet Signed-off-by: Linus Torvalds --- diff --git a/mm/slub.c b/mm/slub.c index 4b3895cb90..74c65af0a5 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -149,13 +149,6 @@ static inline void ClearSlabDebug(struct page *page) /* Enable to test recovery from slab corruption on boot */ #undef SLUB_RESILIENCY_TEST -/* - * Currently fastpath is not supported if preemption is enabled. - */ -#if defined(CONFIG_FAST_CMPXCHG_LOCAL) && !defined(CONFIG_PREEMPT) -#define SLUB_FASTPATH -#endif - #if PAGE_SHIFT <= 12 /* @@ -1514,11 +1507,7 @@ static void *__slab_alloc(struct kmem_cache *s, { void **object; struct page *new; -#ifdef SLUB_FASTPATH - unsigned long flags; - local_irq_save(flags); -#endif if (!c->page) goto new_slab; @@ -1541,9 +1530,6 @@ load_freelist: unlock_out: slab_unlock(c->page); stat(c, ALLOC_SLOWPATH); -#ifdef SLUB_FASTPATH - local_irq_restore(flags); -#endif return object; another_slab: @@ -1575,9 +1561,7 @@ new_slab: c->page = new; goto load_freelist; } -#ifdef SLUB_FASTPATH - local_irq_restore(flags); -#endif + /* * No memory available. * @@ -1619,34 +1603,6 @@ static __always_inline void *slab_alloc(struct kmem_cache *s, { void **object; struct kmem_cache_cpu *c; - -/* - * The SLUB_FASTPATH path is provisional and is currently disabled if the - * kernel is compiled with preemption or if the arch does not support - * fast cmpxchg operations. There are a couple of coming changes that will - * simplify matters and allow preemption. Ultimately we may end up making - * SLUB_FASTPATH the default. - * - * 1. The introduction of the per cpu allocator will avoid array lookups - * through get_cpu_slab(). A special register can be used instead. - * - * 2. The introduction of per cpu atomic operations (cpu_ops) means that - * we can realize the logic here entirely with per cpu atomics. The - * per cpu atomic ops will take care of the preemption issues. - */ - -#ifdef SLUB_FASTPATH - c = get_cpu_slab(s, raw_smp_processor_id()); - do { - object = c->freelist; - if (unlikely(is_end(object) || !node_match(c, node))) { - object = __slab_alloc(s, gfpflags, node, addr, c); - break; - } - stat(c, ALLOC_FASTPATH); - } while (cmpxchg_local(&c->freelist, object, object[c->offset]) - != object); -#else unsigned long flags; local_irq_save(flags); @@ -1661,7 +1617,6 @@ static __always_inline void *slab_alloc(struct kmem_cache *s, stat(c, ALLOC_FASTPATH); } local_irq_restore(flags); -#endif if (unlikely((gfpflags & __GFP_ZERO) && object)) memset(object, 0, c->objsize); @@ -1698,11 +1653,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page, void **object = (void *)x; struct kmem_cache_cpu *c; -#ifdef SLUB_FASTPATH - unsigned long flags; - - local_irq_save(flags); -#endif c = get_cpu_slab(s, raw_smp_processor_id()); stat(c, FREE_SLOWPATH); slab_lock(page); @@ -1734,9 +1684,6 @@ checks_ok: out_unlock: slab_unlock(page); -#ifdef SLUB_FASTPATH - local_irq_restore(flags); -#endif return; slab_empty: @@ -1749,9 +1696,6 @@ slab_empty: } slab_unlock(page); stat(c, FREE_SLAB); -#ifdef SLUB_FASTPATH - local_irq_restore(flags); -#endif discard_slab(s, page); return; @@ -1777,34 +1721,6 @@ static __always_inline void slab_free(struct kmem_cache *s, { void **object = (void *)x; struct kmem_cache_cpu *c; - -#ifdef SLUB_FASTPATH - void **freelist; - - c = get_cpu_slab(s, raw_smp_processor_id()); - debug_check_no_locks_freed(object, s->objsize); - do { - freelist = c->freelist; - barrier(); - /* - * If the compiler would reorder the retrieval of c->page to - * come before c->freelist then an interrupt could - * change the cpu slab before we retrieve c->freelist. We - * could be matching on a page no longer active and put the - * object onto the freelist of the wrong slab. - * - * On the other hand: If we already have the freelist pointer - * then any change of cpu_slab will cause the cmpxchg to fail - * since the freelist pointers are unique per slab. - */ - if (unlikely(page != c->page || c->node < 0)) { - __slab_free(s, page, x, addr, c->offset); - break; - } - object[c->offset] = freelist; - stat(c, FREE_FASTPATH); - } while (cmpxchg_local(&c->freelist, freelist, object) != freelist); -#else unsigned long flags; local_irq_save(flags); @@ -1818,7 +1734,6 @@ static __always_inline void slab_free(struct kmem_cache *s, __slab_free(s, page, x, addr, c->offset); local_irq_restore(flags); -#endif } void kmem_cache_free(struct kmem_cache *s, void *x)