b = p; /* BUG: Compiler and CPU can both reorder!!! */
Finally, the READ_ONCE_CTRL() includes an smp_read_barrier_depends()
-that DEC Alpha needs in order to respect control depedencies.
+that DEC Alpha needs in order to respect control depedencies. Alternatively
+use one of atomic{,64}_read_ctrl().
So don't leave out the READ_ONCE_CTRL().
In summary:
- (*) Control dependencies must be headed by READ_ONCE_CTRL().
- Or, as a much less preferable alternative, interpose
- smp_read_barrier_depends() between a READ_ONCE() and the
+ (*) Control dependencies must be headed by READ_ONCE_CTRL(),
+ atomic{,64}_read_ctrl(). Or, as a much less preferable alternative,
+ interpose smp_read_barrier_depends() between a READ_ONCE() and the
control-dependent write.
(*) Control dependencies can order prior loads against later stores.
and WRITE_ONCE() can help to preserve the needed conditional.
(*) Control dependencies require that the compiler avoid reordering the
- dependency into nonexistence. Careful use of READ_ONCE_CTRL()
- or smp_read_barrier_depends() can help to preserve your control
- dependency. Please see the Compiler Barrier section for more
- information.
+ dependency into nonexistence. Careful use of READ_ONCE_CTRL(),
+ atomic{,64}_read_ctrl() or smp_read_barrier_depends() can help to
+ preserve your control dependency. Please see the Compiler Barrier
+ section for more information.
(*) Control dependencies pair normally with other types of barriers.
explicit lock operations, described later). These include:
xchg();
- cmpxchg();
atomic_xchg(); atomic_long_xchg();
- atomic_cmpxchg(); atomic_long_cmpxchg();
atomic_inc_return(); atomic_long_inc_return();
atomic_dec_return(); atomic_long_dec_return();
atomic_add_return(); atomic_long_add_return();
test_and_clear_bit();
test_and_change_bit();
- /* when succeeds (returns 1) */
+ /* when succeeds */
+ cmpxchg();
+ atomic_cmpxchg(); atomic_long_cmpxchg();
atomic_add_unless(); atomic_long_add_unless();
These are used for such things as implementing ACQUIRE-class and RELEASE-class