From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754463AbYKMO2w (ORCPT ); Thu, 13 Nov 2008 09:28:52 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754127AbYKMO2m (ORCPT ); Thu, 13 Nov 2008 09:28:42 -0500 Received: from bombadil.infradead.org ([18.85.46.34]:57513 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754052AbYKMO2k (ORCPT ); Thu, 13 Nov 2008 09:28:40 -0500 Subject: [PATCH] slab: document SLAB_DESTROY_BY_RCU From: Peter Zijlstra To: Andrew Morton Cc: linux-kernel , hugh , Paul E McKenney , Jens Axboe , Eric Dumazet Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Thu, 13 Nov 2008 15:28:29 +0100 Message-Id: <1226586509.7685.4929.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.24.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Subject: slab: document SLAB_DESTROY_BY_RCU From: Peter Zijlstra Date: Wed, 02 Apr 2008 14:13:42 +0200 Explain this SLAB_DESTROY_BY_RCU thing... Signed-off-by: Peter Zijlstra Acked-by: Jens Axboe Acked-by: Paul E. McKenney --- include/linux/slab.h | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) Index: linux-2.6/include/linux/slab.h =================================================================== --- linux-2.6.orig/include/linux/slab.h +++ linux-2.6/include/linux/slab.h @@ -23,6 +23,32 @@ #define SLAB_CACHE_DMA 0x00004000UL /* Use GFP_DMA memory */ #define SLAB_STORE_USER 0x00010000UL /* DEBUG: Store the last owner for bug hunting */ #define SLAB_PANIC 0x00040000UL /* Panic if kmem_cache_create() fails */ +/* + * SLAB_DESTROY_BY_RCU - **WARNING** READ THIS! + * + * This delays freeing the SLAB page by a grace period, it does _NOT_ + * delay object freeing. This means that if you do kmem_cache_free() + * that memory location is free to be reused at any time. Thus it may + * be possible to see another object there in the same RCU grace period. + * + * This feature only ensures the memory location backing the object + * stays valid, the trick to using this is relying on an independent + * object validation pass. Something like: + * + * rcu_read_lock() + * again: + * obj = lockless_lookup(key); + * if (obj) { + * if (!try_get_ref(obj)) // might fail for free objects + * goto again; + * + * if (obj->key != key) { // not the object we expected + * put_ref(obj); + * goto again; + * } + * } + * rcu_read_unlock(); + */ #define SLAB_DESTROY_BY_RCU 0x00080000UL /* Defer freeing slabs to RCU */ #define SLAB_MEM_SPREAD 0x00100000UL /* Spread some memory over cpuset */ #define SLAB_TRACE 0x00200000UL /* Trace allocations and frees */