lkml.org 
[lkml]   [2008]   [Apr]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
SubjectRe: kmemcheck caught read from freed memory (cfq_free_io_context)
From
Date
On Wed, 2008-04-02 at 13:53 +0200, Jens Axboe wrote:

> > Yeah, SLAB_DESTROY_BY_RCU should have a _HUGE_ comment explaining it,
> > I'm sure this is not the first (nor the last) time people get that
> > wrong.
>
> It should, SLAB_DESTROY_BY_RCU is definitely useful, but it is expected
> to be an 'easier' way of doing the call_rcu() manually. So it definitely
> needs more documentation.
>

Ok I gave it a go, how bad is this text?

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
diff --git a/include/linux/slab.h b/include/linux/slab.h
index f950a89..e049ddc 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -25,6 +25,32 @@
#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 */



\
 
 \ /
  Last update: 2008-04-02 14:17    [from the cache]
©2003-2011 Jasper Spaans