On Tue, 6 Dec 2005, Oliver Neukum wrote:
> Am Dienstag, 6. Dezember 2005 21:13 schrieb Eduardo Pereira Habkost:
> > Anyway, I don't see yet why the atomic_t would make the code slower on
> > non-smp. Is atomic_add_unless(v, 1, 1) supposed to be slower than
> > 'if (!v) v = 1;' ?
>
> spin_lock() can be dropped on UP. atomic_XXX must either use an operation
> on main memory, meaning less efficient code generation, or must disable
> interrupts even on UP.
atomic_add_unless is sort of a special case. It doesn't have a clean
simple implementation, unlike most of the other atomic_t operations. If
an equivalent result could be obtained using, e.g., atomic_inc_and_test,
that would be a better approach.
On the other hand, Oliver needs to be careful about claiming too much. In
general atomic_t operations _are_ superior to the spinlock approach. If
they weren't, atomic_t wouldn't belong in the kernel at all.
Alan Stern
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