Re: [PATCH] radix-tree: fix radix_tree_replace_slot

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Tue, Aug 22, 2006 at 04:25:17PM -0400, Lee Schermerhorn wrote:
> Andrew:
> 
> I was waiting to hear from Nick on this, but I understand that he has
> severely injured one hand, restricting his keyboard access for a while. 
> 
> Christoph has been too swamped to take a look, either.   So, having
> tested it myself, I'll send it on.
> 
> Paul:
> 
> could you take a look at this one.  Also, I'll be sending another
> rcu-radix-tree "cleanup" patch that I'd like your opinion on.

Color me blind -- I certainly missed this one earlier.

Yes, the original is certainly not very functional...

						Thanx, Paul

> Lee
> 
> Fix radix tree direct slot replacement - 2.6.18-rc4-mm2
> 
> radix_tree_replace_slot() was assigning to local variable 'slot'
> instead of to where pslot pointed.  Changed to directly replace
> location pointed to by argument pslot.
> 
> Added comments specifying required locking.
> 
> Note that we do not need to rcu_dereference() the slot to
> obtain the direct pointer flag, as we hold the tree write locked.
> 
> Fixes the migration corruption that we were seeing since the
> rcu-radix-tree patches went in.  With this patch, we can back out
> page-migration-replace-radix_tree_lookup_slot-with-radix_tree_lockup.patch
> to use the more efficient direct access to radix tree slot.
> 
> Signed-off-by:  Lee Schermerhorn <lee.schermerhorn@hp.com>
> 
>  include/linux/radix-tree.h |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6.18-rc4-mm2/include/linux/radix-tree.h
> ===================================================================
> --- linux-2.6.18-rc4-mm2.orig/include/linux/radix-tree.h	2006-08-22 14:30:38.000000000 -0400
> +++ linux-2.6.18-rc4-mm2/include/linux/radix-tree.h	2006-08-22 14:36:54.000000000 -0400
> @@ -133,12 +133,15 @@ static inline void *radix_tree_deref_slo
>   * radix_tree_replace_slot	- replace item in a slot
>   * @pslot:	pointer to slot, returned by radix_tree_lookup_slot
>   * @item:	new item to store in the slot.
> + *
> + * For use with radix_tree_lookup_slot().  Caller must hold tree write locked
> + * across slot lookup and replacement.
>   */
>  static inline void radix_tree_replace_slot(void *pslot, void *item)
>  {
>  	void *slot = *(void **)pslot;
>  	BUG_ON(radix_tree_is_direct_ptr(item));
> -	rcu_assign_pointer(slot,
> +	rcu_assign_pointer(*(void **)pslot,
>  		(void *)((unsigned long)item |
>  			((unsigned long)slot & RADIX_TREE_DIRECT_PTR)));
>  }
> 
> 

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