From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: memory barrier question Date: Thu, 16 Sep 2010 09:37:08 -0700 Message-ID: <20100916163708.GG2462@linux.vnet.ibm.com> References: <3777.1284638136@redhat.com> <6383.1284647456@redhat.com> <20100916150356.GD2462@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e7.ny.us.ibm.com ([32.97.182.137]:34919 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752748Ab0IPQhM (ORCPT ); Thu, 16 Sep 2010 12:37:12 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Miklos Szeredi Cc: dhowells@redhat.com, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org On Thu, Sep 16, 2010 at 06:06:53PM +0200, Miklos Szeredi wrote: > On Thu, 16 Sep 2010, Paul E. McKenney wrote: > > On Thu, Sep 16, 2010 at 03:30:56PM +0100, David Howells wrote: > > > Miklos Szeredi wrote: > > > > > > > Is the rmb() really needed? > > > > > > > > Take this code from fs/namei.c for example: > > > > > > > > inode = next.dentry->d_inode; > > > > if (!inode) > > > > goto out_dput; > > > > > > > > if (inode->i_op->follow_link) { > > > > > > > > It happily dereferences dentry->d_inode without a barrier after > > > > checking it for non-null, while that d_inode might have just been > > > > initialized on another CPU with a freshly created inode. There's > > > > absolutely no synchornization with that on this side. > > > > > > Perhaps it's not necessary; once set, how likely is i_op to be changed once > > > I_NEW is cleared? > > > > Are the path_get()s protecting this? > > No, when creating a file the dentry will go from negative to positive > independently from lookup. The dentry can get instantiated with an > inode between the path_get() and dereferencing ->d_inode. > > > If there is no protection, then something like rcu_dereference() is > > needed for the assignment from next.dentry->d_inode. > > Do I understand correctly that the problem is that a CPU may have a > stale cache associated with *inode, one that was loaded before the > write barrier took effect? Yes, especially if the compiler is aggressively optimizing. > Funny that such a bug could stay unnoticed in so often excercised > code. Yeah I know it's alpha only. I wonder how much of this pattern > exists in the kernel elswhere without the necessary barriers. The probability of failure is low, but non-zero. :-/ Thanx, Paul