Am 24.11.2014 um 19:35 schrieb Linus Torvalds: > On Mon, Nov 24, 2014 at 10:02 AM, Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: >> >> If the goal is to catch non-scalar users, the following is shorter: >> #define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x)) > > Me likey. It probably works well in practice, although I think > > - the "(typeof(x))0)" seems unnecessary and wrong. Why not just "0"? > The typeof is not just longer, but it is incorrect for pointer types > (you can add 0 to a pointer, but you cannot add two pointers together) > > - it does mean that the resulting type ends up being upgraded to > "int", for the usual C type reasons. > > Note that the "upgraded to 'int'" is true with or without the > "(typeof(x))0". If you add two 'char' values, the addition is still > done in 'int'. > > Maybe you *meant* that typeof to fix the second problem, like so: > > (typeof(x)) (0 + *(volatile typeof(x) *)&(x)) > > Hmm? That casts the result of the addition, not the zero. Looks really nice, but does not work with ACCESS_ONCE is on the left-hand side: include/linux/rculist.h: In function 'hlist_add_before_rcu': ./arch/x86/include/asm/barrier.h:127:18: error: lvalue required as left operand of assignment ACCESS_ONCE(*p) = (v); \ Alexei's variant is also broken: include/linux/cgroup.h: In function 'task_css': include/linux/compiler.h:381:40: error: invalid operands to binary + (have 'struct css_set *' and 'struct css_set * volatile') #define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x)) Anyone with a new propopal? ;-) ^ Christian
