linux-kernel - Re: [PATCH] driver core: fix shutdown races with probe/remove

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Date:	Wed, 6 Jun 2012 12:05:08 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Ming Lei <ming.lei@...onical.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] driver core: fix shutdown races with probe/remove

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

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