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

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Date:	Wed, 6 Jun 2012 11:44:50 -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:

> > That just seems wrong.  By the same reasoning, the compiler is within 
> > its rights to transform either the original code or the code using 
> > ACCESS_ONCE into:
> > 
> > 	b = 999;
> > 	if (a)
> > 		b = 9;
> > 	else
> > 		b = 42;
> > 
> > and again, other code would be confused.  The simple fact is that 
> > SMP-safe code is not likely to be produced by a compiler that assumes 
> > everything is single-threaded.
> 
> If you use ACCESS_ONCE(), the compiler is prohibited from inserting
> the "b = 999".

What prohibits it?

>  If you don't use ACCESS_ONCE(), the compiler really
> is permitted to insert the "b = 999".  So, why would the compiler do
> such a thing?  One possible reason would be from optimizations using
> large registers to hold multiple values.  A store from such a register
> could clobber unrelated variables, but as long as the compiler fixes
> up the clobbering after the fact, it is within its rights to do so.
> 
> The sad fact is that the C standard really does permit the compiler
> to assume that it is generating sequential code.

Compiling the kernel requires quite a few extensions to the C standard.  
Assumptions about generating sequential code may well be among them.

Alan Stern

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