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Date: Wed, 13 May 2020 22:51:53 -0400 (EDT) From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> To: Thomas Gleixner <tglx@...utronix.de> Cc: rostedt <rostedt@...dmis.org>, linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>, paulmck <paulmck@...nel.org>, Andy Lutomirski <luto@...nel.org>, Alexandre Chartre <alexandre.chartre@...cle.com>, Frederic Weisbecker <frederic@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <sean.j.christopherson@...el.com>, Masami Hiramatsu <mhiramat@...nel.org>, Petr Mladek <pmladek@...e.com>, "Joel Fernandes, Google" <joel@...lfernandes.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, Brian Gerst <brgerst@...il.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [patch V4 part 1 05/36] x86/entry: Flip _TIF_SIGPENDING and _TIF_NOTIFY_RESUME handling ----- On May 13, 2020, at 8:12 PM, Thomas Gleixner tglx@...utronix.de wrote: [...] > >>> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote: > >>> Also, color me confused: is "do_signal()" actually running any user-space, >>> or just setting up the user-space stack for eventual return to signal >>> handler ? > > I'm surprised that you can't answer that question yourself. How did you > ever make rseq work and how did rseq_signal_deliver() end up in > setup_rt_frame()? > > Hint: Tracing might answer that question > > And to cut it short: > > Exit to user space happnes only through ONE channel, i.e. leaving > prepare_exit_to usermode(). > [...] Yes, I'm very well aware of this. But the patch commit message states: "Make sure task_work runs before any kind of userspace -- very much including signals -- is invoked." which seems to imply that "userspace" can be "invoked" before the task_work runs. Which makes no sense whatsoever. Hence my confused state. >>> Also, it might be OK, but we're changing the order of two things which >>> have effects on each other: restartable sequences abort fixup for preemption >>> and do_signal(), which also have effects on rseq abort. >>> >>> Because those two will cause the abort to trigger, I suspect changing >>> the order might be OK, but we really need to think this through. > > That's a purely academic problem. The order is completely > irrelevant. You have to handle any order anyway: Yes indeed, whether either a signal handler frame fixup or return IP fixup fires first (clearing the rseq_cs pointer in the process) is irrelevant, because they will have the effect on the user-space program's flow. And as you say, given it is run in a loop and can be preempted, any order can happen here, so we have to be prepared for it. This loop has caused me tons of headaches when stress-testing on NUMA machines by the way. > That said, even for the case Andy and Peter were looking at (MCE) the > ordering is completely irrelevant. Not sure about that, see Andy's follow up. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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