On Wed, 4 Dec 2013 14:46:59 -0800
"Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
>
> The situations in which ACCESS_ONCE() is required are not well documented,
> so this commit adds some verbiage to memory-barriers.txt.
[...]
> + But please note that the compiler is also closely watching what you
> + do with the value after the ACCESS_ONCE(). For example, suppose you
> + do the following and MAX is a preprocessor macro with the value 1:
> +
> + for ((tmp = ACCESS_ONCE(a)) % MAX)
> + do_something_with(tmp);
That sure looks like it was meant to be "while" instead of "for"?
[...]
> + (*) The compiler is within its rights to reorder memory accesses unless
> + you tell it not to. For example, consider the following interaction
> + between process-level code and an interrupt handler:
> +
> + void process_level(void)
> + {
> + msg = get_message();
> + flag = true;
> + }
> +
> + void interrupt_handler(void)
> + {
> + if (flag)
> + process_message(msg);
> + }
> +
> + There is nothing to prevent the the compiler from transforming
> + process_level() to the following, in fact, this might well be a
> + win for single-threaded code:
> +
> + void process_level(void)
> + {
> + flag = true;
> + msg = get_message();
> + }
> +
> + If the interrupt occurs between these two statement, then
> + interrupt_handler() might be passed a garbled msg. Use ACCESS_ONCE()
> + to prevent this as follows:
> +
> + void process_level(void)
> + {
> + ACCESS_ONCE(msg) = get_message();
> + ACCESS_ONCE(flag) = true;
> + }
> +
> + void interrupt_handler(void)
> + {
> + if (ACCESS_ONCE(flag))
> + process_message(ACCESS_ONCE(msg));
> + }
Looking at this, I find myself wondering why you couldn't just put a
barrier() between the two statements in process_level()? ACCESS_ONCE()
seems like a heavy hammer to just avoid reordering of two assignments.
What am I missing, and what could be added here to keep the other folks as
dense as me from missing the same thing?
Thanks,
jon
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