From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753913AbcFGA64 (ORCPT ); Mon, 6 Jun 2016 20:58:56 -0400 Received: from mail-oi0-f52.google.com ([209.85.218.52]:35452 "EHLO mail-oi0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753057AbcFGA6z (ORCPT ); Mon, 6 Jun 2016 20:58:55 -0400 MIME-Version: 1.0 In-Reply-To: <20160607004058.GH14480@ZenIV.linux.org.uk> References: <5755D671.9070908@intel.com> <20160606211522.GF14480@ZenIV.linux.org.uk> <20160606220753.GG14480@ZenIV.linux.org.uk> <20160607004058.GH14480@ZenIV.linux.org.uk> From: Linus Torvalds Date: Mon, 6 Jun 2016 17:58:53 -0700 X-Google-Sender-Auth: Q5dnShJhvTfdLLEiUs4XXlFa-1I Message-ID: Subject: Re: performance delta after VFS i_mutex=>i_rwsem conversion To: Al Viro Cc: Dave Hansen , "Chen, Tim C" , Ingo Molnar , Davidlohr Bueso , "Peter Zijlstra (Intel)" , Jason Low , Michel Lespinasse , "Paul E. McKenney" , Waiman Long , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 6, 2016 at 5:40 PM, Al Viro wrote: > > static struct dentry *next_positive(struct dentry *parent, > struct dentry *child, int count) > { > struct list_head *p = child ? &child->d_child : &parent->d_subdirs; >>From your description, you seem to be very confused about what "child == NULL" means. Here it means that it's a cursor to the beginning, but in your commentary on move_cursor(), you say "moves cursor immediately past child *or* to the very end if child is NULL". That's very confusing. Is NULL beginning or end? I really think you'd be better off having a special ERR_PTR value for end, possibly as a flag value in the cursor dentry. The whole "what does NULL mean" confusion exists inside that "next_positive" too: > unsigned *seq = &parent->d_inode->i_dir_seq, n; > do { > int i = count; > n = smp_load_acquire(seq) & ~1; > rcu_read_lock(); > do { > p = p->next; > if (p == &parent->d_subdirs) { > child = NULL; > break; > } look, here you return NULL for "end" again. Even though it meant beginning at the start of the function. Nasty. Also, may I suggest that there is a very trivial special case for "next_positive()" that needs no barriers or sequence checking or anything else: at the very beginning, just load the "->next" pointer, and if it's a positive entry, you're done. That's going to be the common case when there _isn't_ crazy multi-threaded readdirs going on, so it's worth handling separately. In fact, if you have a special value for the case of "cursor is at end" situation, then for the small directory case that can be handled with a single getdents call, you'll *never* set the cursor in the child list at all, which means that the above special case for next_positive() is actually the common case even for the threaded situation. Linus