From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by zproxy.gmail.com with SMTP id l1so1318255nzf for ; Mon, 13 Mar 2006 07:24:47 -0800 (PST) Message-ID: <661de9470603130724mc95405dr6ee32d00d800d37@mail.gmail.com> Date: Mon, 13 Mar 2006 20:54:47 +0530 From: "Balbir Singh" Subject: Re: [patch 1/3] radix tree: RCU lockless read-side In-Reply-To: <4414E2CB.7060604@yahoo.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Content-Disposition: inline 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> Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Nick Piggin , Linux Kernel , Linux Memory Management List-ID: > > 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? -- 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