Messages in this thread | | | Date | Mon, 24 Nov 2014 12:48:05 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types |
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On Mon, Nov 24, 2014 at 12:29:07PM -0800, Alexei Starovoitov wrote: > On Mon, Nov 24, 2014 at 11:07 AM, Christian Borntraeger > <borntraeger@de.ibm.com> wrote: > > > > Anyone with a new propopal? ;-) ^ > > one more proposal :) > #define __ACCESS_ONCE(x) ({ typeof(x) __var = 0; (volatile typeof(x) *)&(x); }) > #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) > > works as lvalue... > the basic idea is the same: > constant zero can be used to initialize any scalar > (including pointers), but unions and structs will fail to compile as: > "error: invalid initializer" > > If I'm reading pr58145 gcc bug report correctly, it > miscompiles only structs, so we can let ACCESS_ONCE > to work on unions. Then the following will rejects structs only: > #define __ACCESS_ONCE(x) ({ (typeof(x))0; (volatile typeof(x) *)&(x); }) > #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
You beat me to it. ;-)
However, your approach would allow an ACCESS_ONCE() of a long long to compile on 32-bit systems where it has to be broken up into a pair of 32-bit accesses.
Thanx, Paul
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