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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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