🟡 plain
⚡ volatile
🔒 lock
▶ A: done = true
▶ B: check
↺ Reset
Two cores · one RAM
RAM done:
false
· Core2 sees:
false
· spins:
0
🧠
Core 1 · Thread A
cache
done:
false
done = true;
🧠
Core 2 · Thread B
cache
done:
false
while (!done) { spin }
spins: 0
🌀 read hoisted into a register
💾 Main memory (RAM)
done:
false
🔀 Reordering
order:
as written
Thread A writes
① data = 42;
② ready = true;
Thread B sees
data:
0
· ready:
false
⇄ Swap
▶ Run A
Thread B is spinning
B keeps reading
done
from its own cache. Press
▶ A: done = true
.
▸
volatile
buys you
visibility
, not atomicity —
count++
is still a race.