From: Peter Zijlstra Date: Thu, 13 Nov 2008 18:40:12 +0000 (+0200) Subject: slab: document SLAB_DESTROY_BY_RCU X-Git-Tag: v2.6.28-rc5~20^2 X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=d7de4c1dc3a2faca0bf05d9e342f885cb2696766;p=linux-2.6-omap-h63xx.git slab: document SLAB_DESTROY_BY_RCU Explain this SLAB_DESTROY_BY_RCU thing... [hugh@veritas.com: add a pointer to comment in mm/slab.c] Signed-off-by: Peter Zijlstra Acked-by: Jens Axboe Acked-by: Paul E. McKenney Acked-by: Christoph Lameter Signed-off-by: Hugh Dickins Signed-off-by: Pekka Enberg --- diff --git a/include/linux/slab.h b/include/linux/slab.h index ba965c84ae0..000da12b5cf 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -23,6 +23,34 @@ #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(); + * + * See also the comment on struct slab_rcu in mm/slab.c. + */ #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 */