From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4415F410.90706@yahoo.com.au> Date: Tue, 14 Mar 2006 09:37:04 +1100 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [patch 1/3] radix tree: RCU lockless read-side References: <20060207021822.10002.30448.sendpatchset@linux.site> <20060207021831.10002.84268.sendpatchset@linux.site> <661de9470603110022i25baba63w4a79eb543c5db626@mail.gmail.com> <44128EDA.6010105@yahoo.com.au> <661de9470603121904h7e83579boe3b26013f771c0f2@mail.gmail.com> <4414E2CB.7060604@yahoo.com.au> <661de9470603130724mc95405dr6ee32d00d800d37@mail.gmail.com> In-Reply-To: <661de9470603130724mc95405dr6ee32d00d800d37@mail.gmail.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Balbir Singh Cc: Nick Piggin , Linux Kernel , Linux Memory Management List-ID: Balbir Singh wrote: > > >>But we should have already rcu_dereference()ed "slot", right >>(in the loop above this one)? That means we are now able to >>dereference it, and the data at the other end will be valid. >> >> > >Yes, but my confusion is about the following piece of code > > > > for ( ; height > 1; height--) { > > for (i = (index >> shift) & RADIX_TREE_MAP_MASK ; > i < RADIX_TREE_MAP_SIZE; i++) { >- if (slot->slots[i] != NULL) >+ __s = rcu_dereference(slot->slots[i]); >+ if (__s != NULL) > break; > index &= ~((1UL << shift) - 1); > index += 1UL << shift; >@@ -531,14 +550,14 @@ __lookup(struct radix_tree_root *root, v > goto out; > > shift -= RADIX_TREE_MAP_SHIFT; >- slot = slot->slots[i]; >+ slot = __s; > } > > /* Bottom level: grab some items */ > for (i = index & RADIX_TREE_MAP_MASK; i < RADIX_TREE_MAP_SIZE; i++) { > index++; > if (slot->slots[i]) { >- results[nr_found++] = slot->slots[i]; >+ results[nr_found++] = &slot->slots[i]; > if (nr_found == max_items) > goto out; > } > > >In the for loop, lets say __s is *not* NULL, we break from the loop. >In the loop below >slot->slots[i] is derefenced without rcu, __s is not used. Is that not >inconsistent? > > The "slots" member is an array, not an RCU assigned pointer. As such, after doing rcu_dereference(slot), you can access slot->slots[i] without further memory barriers I think? But I agree that code now is a bit inconsistent. I've cleaned things up a bit in my tree now... but perhaps it is easier if you send a patch to show what you mean (because sometimes I'm a bit dense, I'm afraid). Thanks, Nick -- Send instant messages to your online friends http://au.messenger.yahoo.com -- 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