matthew (nobody.delete@this.example.com) on August 19, 2018 7:58 pm wrote:
>
> Solaris had the ability for a thread to tell the kernel it was holding a mutex and that it shouldn't
> be preempted until it had dropped the lock. There have been several attempts to add something
> like that to Linux, but none have succeeded yet. Nothing with hardware assists either.

Linux now (merged into the latest released kernel version, 4.18) actually has what could be seen as the reverse of that: "rseq" aka restartable sequences.

It doesn't disable preemption (which is crazy and all kinds of stupid), but it does allow user space to see if it has been preempted, and mark certain sequences to be done atomically. And if preemption happens, the sequence gets aborted.

So you can think of rseq as kind of like the OS equivalent of transactional memory, but instead of the transactional sequence being aborted on a cache conflict, it gets aborted on preemption.

That allows you to do certain per-cpu things in user space (as opposed to per-thread).

And that, in turn, can be a big deal when you have 4 cores, but 4 million threads. You don't want to have the memory overhead of per-thread allocations, when all you really wanted was the cache advantages of per-cpu counters.

It's a pretty limited use-case, and I don't expect normal users to really ever see it. But it is designed to allow for things like per-cpu malloc libraries etc, and a few other very specific situations where you can take advantage of it.

We'll see if people end up taking advantage of it. The downside with a lot of clever interfaces is that because they are non-standard, you really don't see people using them unless there is a big win or unless you can transparently hide them in a library with absolutely zero downside from the portable standard approach.

Which is basically what seems to have killed transactional memory. The library approach (HLE) ends up performing horribly in many real-life situations, so it's not possible to use as a direct transparent replacement, and the full transactional model is so non-portable that it's not worth spending effort on.

We'll see if rseq can do better.

Linus