From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: [PATCH] radix-tree: cleanup radix_tree_deref_slot() and _lookup_slot() comments From: Lee Schermerhorn Content-Type: text/plain Date: Tue, 22 Aug 2006 16:32:52 -0400 Message-Id: <1156278772.5622.23.camel@localhost> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton , "Paul E. McKenney" Cc: Nick Piggin , Christoph Lameter , linux-mm List-ID: '_deref_slot() function, and adds more explanation of expected/required locking to the direct slot access functions. I separated it out, because it doesn't fix a serious bug, like the previous one. Paul: do you agree that we don't need rcu_dereference() in the _deref_slot() as it can only be used while the tree is held [probably write] locked? Do the comments look OK? Lee Cleanup radix tree slot dereference and lookup comments - 2.6.18-rc4-mm2 radix_tree_deref_slot() was actually dereferencing the pointer in the assignment to the local variable 'slot' and then rcu_dereference()ing the results of an expression [return value of an inline function]. Because we must hold the tree locked across lookup_slot() and _deref_slot(), we don't need the rcu_dereference() at all. Added comments specifying required locking for _lookup_slot() and _deref_slot(). Signed-off-by: Lee Schermerhorn include/linux/radix-tree.h | 9 +++++++-- lib/radix-tree.c | 5 ++++- 2 files changed, 11 insertions(+), 3 deletions(-) Index: linux-2.6.18-rc4-mm2/lib/radix-tree.c =================================================================== --- linux-2.6.18-rc4-mm2.orig/lib/radix-tree.c 2006-08-22 14:47:06.000000000 -0400 +++ linux-2.6.18-rc4-mm2/lib/radix-tree.c 2006-08-22 14:48:01.000000000 -0400 @@ -336,7 +336,10 @@ EXPORT_SYMBOL(radix_tree_insert); * @root. This is useful for update-if-exists operations. * * This function cannot be called under rcu_read_lock, it must be - * excluded from writers, as must the returned slot. + * excluded from writers, as must the returned slot for subsequent + * use by radix_tree_deref_slot() and radix_tree_replace slot. + * Caller must hold tree write locked across slot lookup and + * replace. */ void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long index) { 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 14:47:45.000000000 -0400 +++ linux-2.6.18-rc4-mm2/include/linux/radix-tree.h 2006-08-22 14:48:54.000000000 -0400 @@ -122,12 +122,17 @@ do { \ /** * radix_tree_deref_slot - dereference a slot * @pslot: pointer to slot, returned by radix_tree_lookup_slot - * @returns: item that was stored in that slot. + * @returns: item that was stored in that slot with any direct pointer flag + * removed. + * + * For use with radix_tree_lookup_slot(). Caller must hold tree at least read + * locked across slot lookup and dereference. More likely, will be used with + * radix_tree_replace_slot(), as well, so caller will hold tree write locked. */ static inline void *radix_tree_deref_slot(void *pslot) { void *slot = *(void **)pslot; - return rcu_dereference(radix_tree_direct_to_ptr(slot)); + return radix_tree_direct_to_ptr(slot); } /** * radix_tree_replace_slot - replace item in a slot -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org