Control dependency between prior load in while condition and later store?
From: Daniel Jordan
Date: Wed Apr 04 2018 - 15:30:27 EST
A question for memory-barriers.txt aficionados.
Is there a control dependency between the prior load of 'a' and the
later store of 'c'?:
while (READ_ONCE(a));
WRITE_ONCE(c, 1);
I have my doubts because memory-barriers.txt doesn't talk much about
loops and because of what that document says here:
In addition, control dependencies apply only to the then-clause and
else-clause of the if-statement in question. In particular, it does
not necessarily apply to code following the if-statement:
q = READ_ONCE(a);
if (q) {
WRITE_ONCE(b, 1);
} else {
WRITE_ONCE(b, 2);
}
WRITE_ONCE(c, 1); /* BUG: No ordering against the read from 'a'. */
It's not obvious to me how the then-clause/else-clause idea maps onto
loops, but if we think of the example at the top like this...
while (1) {
if (!READ_ONCE(a)) {
WRITE_ONCE(c, 1);
break;
}
}
...then the dependent store is within the then-clause. Viewed this way,
it seems there would be a control dependency between a and c.
Is that right?
Thanks,
Daniel