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Date: Tue, 19 May 2020 22:20:02 +0200 From: Thomas Gleixner <tglx@...utronix.de> To: Andy Lutomirski <luto@...nel.org> Cc: LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>, "Paul E. McKenney" <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>, Steven Rostedt <rostedt@...dmis.org>, Joel Fernandes <joel@...lfernandes.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, Brian Gerst <brgerst@...il.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Will Deacon <will@...nel.org>, Tom Lendacky <thomas.lendacky@....com>, Wei Liu <wei.liu@...nel.org>, Michael Kelley <mikelley@...rosoft.com>, Jason Chen CJ <jason.cj.chen@...el.com>, Zhao Yakui <yakui.zhao@...el.com>, "Peter Zijlstra \(Intel\)" <peterz@...radead.org> Subject: Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() Thomas Gleixner <tglx@...utronix.de> writes: > Andy Lutomirski <luto@...nel.org> writes: >> On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@...utronix.de> wrote: >>> The pagefault handler cannot use the regular idtentry_enter() because that >>> invokes rcu_irq_enter() if the pagefault was caused in the kernel. Not a >>> problem per se, but kernel side page faults can schedule which is not >>> possible without invoking rcu_irq_exit(). >>> >>> Adding rcu_irq_exit() and a matching rcu_irq_enter() into the actual >>> pagefault handling code would be possible, but not pretty either. >>> >>> Provide idtentry_entry/exit_cond_rcu() which calls rcu_irq_enter() only >>> when RCU is not watching. The conditional RCU enabling is a correctness >>> issue: A kernel page fault which hits a RCU idle reason can neither >>> schedule nor is it likely to survive. But avoiding RCU warnings or RCU side >>> effects is at least increasing the chance for useful debug output. >>> >>> The function is also useful for implementing lightweight reschedule IPI and >>> KVM posted interrupt IPI entry handling later. >> >> Why is this conditional? That is, couldn't we do this for all >> idtentry_enter() calls instead of just for page faults? Evil things >> like NMI shouldn't go through this path at all. > > I thought about that, but then ended up with the conclusion that RCU > might be unhappy, but my conclusion might be fundamentally wrong. It's about this: rcu_nmi_enter() { if (!rcu_is_watching()) { make it watch; } else if (!in_nmi()) { do_magic_nohz_dyntick_muck(); } So if we do all irq/system vector entries conditional then the do_magic() gets never executed. After that I got lost... Thanks, tglx
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