Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757189Ab2FFQFO (ORCPT ); Wed, 6 Jun 2012 12:05:14 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:51302 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1756782Ab2FFQFJ (ORCPT ); Wed, 6 Jun 2012 12:05:09 -0400 Date: Wed, 6 Jun 2012 12:05:08 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: "Paul E. McKenney" cc: Ming Lei , Greg Kroah-Hartman , USB list , Kernel development list Subject: Re: [PATCH] driver core: fix shutdown races with probe/remove In-Reply-To: <20120606154808.GK19601@linux.vnet.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2175 Lines: 71 On Wed, 6 Jun 2012, Paul E. McKenney wrote: > On Wed, Jun 06, 2012 at 11:21:52AM -0400, Alan Stern wrote: > > On Wed, 6 Jun 2012, Paul E. McKenney wrote: > > > > > No sane compiler would change it to a byte-at-a-time store, but the > > > compiler would nevertheless be within its rights to do so. And a quick > > > review of certain LKML threads could easily cause anyone to question gcc's > > > sanity. Furthermore, the compiler is permitted to make transformations > > > like the following, which it might well do to save a branch: > > > > > > if (b) a = 0; > > > a = 1; if (b) > > > else a = 1; > > > a = 0; > > > > The compiler would be forbidden if the original code were > > > > if (b) > > ACCESS_ONCE(a) = 1; > > else > > ACCESS_ONCE(a) = 0; > > > > But if I remember correctly, the code snippet we were talking was more > > like: > > > > if (ACCESS_ONCE(b)) > > a = 1; > > > > Isn't this use of ACCESS_ONCE unnecessary? > > That would depend on what else is nearby. Here's the relevant part of the original patch: @@ -467,6 +473,12 @@ EXPORT_SYMBOL_GPL(driver_attach); static void __device_release_driver(struct device *dev) { struct device_driver *drv; + int idx; + + idx = srcu_read_lock(&driver_srcu); + + if (ACCESS_ONCE(device_shutdown_started)) + goto exit; drv = dev->driver; if (drv) { @@ -494,6 +506,8 @@ static void __device_release_driver(struct device *dev) dev); } +exit: + srcu_read_unlock(&driver_srcu, idx); } > There are some limitations because volatile accesses are not allowed to > move past "sequence points", but it would be possible to come up with > similar examples. This sort of thing is why C1x has a memory model and > why it allows variables to be designated as needing to be SMP-safe. Almost certainly the kernel won't use this facility. Or else it will just require that _all_ global variables be SMP-safe. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/