----- On Nov 21, 2017, at 12:21 PM, Andi Kleen andi@xxxxxxxxxxxxxx wrote: > On Tue, Nov 21, 2017 at 09:18:38AM -0500, Mathieu Desnoyers wrote: >> Hi, >> >> Following changes based on a thorough coding style and patch changelog >> review from Thomas Gleixner and Peter Zijlstra, I'm respinning this >> series for another RFC. >> > My suggestion would be that you also split out the opv system call. > That seems to be main contention point currently, and the restartable > sequences should be useful without it. I consider rseq to be incomplete and a pain to use in various scenarios without cpu_opv. About the contention point you refer to: Using vDSO as an example of how things should be done is just wrong: the vDSO interaction with debugger instruction single-stepping is broken, as I detailed in my previous email. Thomas' proposal of handling single-stepping with a user-space locking fallback, which is pretty much what I had in 2016, pushes a lot of complexity to user-space, requires an extra branch in the fast-path, as well as additional store-release/load-acquire semantics for consistency. I don't plan going down that route. Other than that, I have not received any concrete alternative proposal to properly handle single-stepping. The only opposition against cpu_opv is that there *should* be an hypothetical simpler solution. The rseq idea is not new: it's been presented by Paul Turner in 2012 at LPC. And so far, cpu_opv is the overall simplest and most efficient way I encountered to handle single-stepping, and it gives extra benefits, as described in my changelog. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
