----- On Apr 16, 2018, at 2:39 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote: > On Mon, Apr 16, 2018 at 11:35 AM, Mathieu Desnoyers > <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> Specifically for single-stepping, the __rseq_table section introduced >> at user-level will allow newer debuggers and tools which do line and >> instruction-level single-stepping to skip over rseq critical sections. >> However, this breaks existing debuggers and tools. > > I really don't think single-stepping is a valid argument. > > Even if the cpu_opv() allows you to "single step", you're not actually > single stepping the same thing that you're using. So you are literally > debugging something else than the real code. > > At that point, you don't need "cpu_opv()", you need to just load > /dev/urandom in a buffer, and single-step that. Ta-daa! No new kernel > functionality needed. > > So if the main argument for cpu_opv is single-stepping, then just rip > it out. It's not useful. No, single-stepping is not the only use-case. Accessing remote cpu data is another use-case fulfilled by cpu_opv, which I think is more compelling. > > Anybody who cares deeply about single-stepping shouldn't be using > optimistic algorithms, and they shouldn't be doing multi-threaded > stuff either. They won't be able to use things like transactional > memory either. > > You can't single-step into the kernel to see what the kernel does > either when you're debugging something. > > News at 11: "single stepping isn't always viable". I don't mind if people cannot stop the program with a debugger and observe the state of registers manually at each step though a rseq critical section. I do mind breaking existing tools that rely on single-stepping approaches to automatically analyze program behavior [1,2]. Introducing a rseq critical section into a library (e.g. glibc memory allocator) would cause existing programs being analyzed with existing tools to hang. And I try very hard to avoid being told I'm the one breaking user-space. ;-) Thanks, Mathieu [1] http://rr-project.org/ [2] https://www.gnu.org/software/gdb/news/reversible.html -- 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
