=============== ===============
{ A == 1, B == 2, C == 3, P == &A, Q == &C }
B = 4; Q = P;
- P = &B D = *Q;
+ P = &B; D = *Q;
There is an obvious data dependency here, as the value loaded into D depends on
the address retrieved from P by CPU 2. At the end of the sequence, any of the
{ A == 1, B == 2, C == 3, P == &A, Q == &C }
B = 4;
<write barrier>
- WRITE_ONCE(P, &B)
+ WRITE_ONCE(P, &B);
Q = READ_ONCE(P);
D = *Q;
and WRITE_ONCE() are more selective: With READ_ONCE() and
WRITE_ONCE(), the compiler need only forget the contents of the
indicated memory locations, while with barrier() the compiler must
- discard the value of all memory locations that it has currented
+ discard the value of all memory locations that it has currently
cached in any machine registers. Of course, the compiler must also
respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
though the CPU of course need not do so.
to issue the loads in the correct order (eg. `a[b]` would have to load
the value of b before loading a[b]), however there is no guarantee in
the C specification that the compiler may not speculate the value of b
-(eg. is equal to 1) and load a before b (eg. tmp = a[1]; if (b != 1)
+(eg. is equal to 1) and load a[b] before b (eg. tmp = a[1]; if (b != 1)
tmp = a[b]; ). There is also the problem of a compiler reloading b after
having loaded a[b], thus having a newer copy of b than a[b]. A consensus
has not yet been reached about these problems, however the READ_ONCE()