[damon_dam]

> Who says you can’t make a library that does both?

Of course you can. I just meant that the linked article didn't.

> On LR/SC: to any atomics experts listening, isn’t it technically “obstruction-free” (as per the Wikipedia definitions at least) rather than lock-free?

The better criterion IMO is loop-free, which makes it a little easier to understand. Consider the following spin-locking code (with overabundant memory barriers):

   do { p = *a; } while (p == 0x1 || !atomic_compare_and_swap(p, 0x1, a));
   memory_barrier();
   // do stuff that looks at *p
   q->next = p;
   memory_barrier();
   atomic_store(q, a);

Here's the equivalent LL/SC version:

   do {
     p = ll(a);
     memory_barrier();
     // do stuff that looks at *p
     q->next = p;
     memory_barrier();
   } while (!sc(q, a));

The pointer-tagging version is also obviously not loop-free. Which is faster, in which cases, and by how much?

The oversimplified answer is that LL/SC is probably slightly faster than spin-locking on most platforms and cases, but pointer-tagging might not be.