On Thu, Dec 29, 2016 at 11:54:02AM +0900, Akira Yokosawa wrote:
> Hi Paul,
>
> As I'm not so familiar with Linux kernel programming,
> I found the following hunk in commit 7945ae1a06c5 difficult to grasp at first glance.
>
> @@ -564,7 +584,7 @@ the other will wait until the first thread releases the lock.
> 38 exit(-1);
> 39 }
> 40 for (i = 0; i < 3; i++) {
> - 41 ACCESS_ONCE(x)++;
> + 41 READ_ONCE(x)++;
> 42 poll(NULL, 0, 5);
> 43 }
> 44 if (pthread_mutex_unlock(pmlp) != 0) {
>
> "The name READ_ONCE() seems to imply it is used for read access, but what
> happens when it is used with a "++" operator?" was what I thought.
>
> "ACCESS_ONCE(x)++" was already somewhat confusing for me.
>
> Once you know its definition, you can see there is no problem. But it still
> looks strange...
>
> Don't kernel programmers feel strangeness in "READ_ONCE(x)++"?
>
> This is just a random question. But if you could add some explanation of the
> usage, that would be of help for novice programmers.
Heh! Good catch, thank you!
This would give a compiler error in the Linux kernel (I think, anyway),
but worked by accident given the definitions perfbook uses.
I have fixed this with your Reported-by.
Thanx, Paul
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