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| author | Peter Zijlstra <a.p.zijlstra@chello.nl> | 2008-11-13 20:40:12 +0200 |
|---|---|---|
| committer | Pekka Enberg <penberg@cs.helsinki.fi> | 2008-11-13 20:49:02 +0200 |
| commit | d7de4c1dc3a2faca0bf05d9e342f885cb2696766 (patch) | |
| tree | c7ea498a84b9319443af8cafba5deb6602dfb3a5 | |
| parent | 02f5621042e3f7e2fb6c741cbe5ee7c1f3caf354 (diff) | |
| download | linux-d7de4c1dc3a2faca0bf05d9e342f885cb2696766.tar.gz | |
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 <a.p.zijlstra@chello.nl> Acked-by: Jens Axboe <jens.axboe@oracle.com> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Christoph Lameter <cl@linux-foundation.org> Signed-off-by: Hugh Dickins <hugh@veritas.com> Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
| -rw-r--r-- | include/linux/slab.h | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/include/linux/slab.h b/include/linux/slab.h index ba965c84ae06..000da12b5cf0 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 */ |