[jstimpfle]

I think it wasn't explained in a very accessible way. If I got the gist right, this essentially brings "per-CPU" synchronization to userland. It's typical in the kernel to have per-cpu data, while per-thread data is rare and typically impractical. There is a high number of threads managed by the kernel, most of which probably belong to a userland process, most of which do not participate in any given synchronisation scheme. Also threads are often too much of an abstraction for parallel programming needs, given that they are hiding for example cache effects. So it's natural to want to use per-cpu data instead of thread_local data in a userland process, I know I've been wishing for that many times.

With rseq, we can allocate in any userland process one instance of a given synchronisation data structure per each CPU. It's important to understand that userland code accessing per-cpu data structures cannot prevent being scheduled away from a CPU and being replaced by another thread (kernel code can block scheduler for short critical sections). Such a replacement thread may subsequently corrupt that same data that was still in the middle of the transaction. But we can make a subset of transactions safe at least: If a transaction gets committed in a single (final) atomic instruction, and we get kernel support for this transaction to be restarted in case there has been a schedule mid-way, this is a guarantee that at the time of commit, the entire transaction hasn't been interrupted by the scheduler. I.e. a kind of "mutual exclusion" guarantee.

Did I get that right?

[saagarjha]

You don't have to use it for this. For example, you can use it for your own transactional memory or hazard pointer scheme.