Really nice, but I have one comment:

> Unlike e.g. C++ atomics in userspace they are unordered or weakly ordered by default in a lot of cases. A lot of people are surprised by that, and then have an even harder time understanding the memory barriers they need to sprinkle over the code to make it work correctly.

C++ atomics are "ordered but not quite": except for memory barriers, C++ seq_cst atomics are only ordered against other seq_cst atomics. The totally-ordered Linux atomics instead are ordered also against READ_ONCE and WRITE_ONCE.

The Linux atomics API is huge and unorthogonal, but the semantics are more pleasant to work with if you really need to. If you do not understand acquire/release or memory barriers, you probably shouldn't be using atomics either, so the complex API is not really a big deal. The most common uses of atomics are wrapped already in the kernel (e.g. kref_get_unless_zero or kref_put_mutex) anyway.

Posted Aug 4, 2022 10:04 UTC (Thu) by epa (subscriber, #39769) [Link]

> In that case your cleanup code needs to somehow deal with these zombies and ensure there’s no confusion, and vice versa any code that resurrects a zombie needs to deal the wooden spikes the cleanup code might throw at an inopportune time.

No worries, spikes is a weapon against vampires and poses no threat to zombies. Had the cleanup code wielded a typical zombie weapon like an axe or machete, the resurrection code would indeed have to tread carefully.