linux-kernel - Re: [patch V4 part 5 07/31] x86/entry: Provide idtentry_entry/exit_cond

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Date:   Mon, 11 May 2020 16:13:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>
Subject: Re: [patch V4 part 5 07/31] x86/entry: Provide
 idtentry_entry/exit_cond_rcu()

On Mon, May 11, 2020 at 03:53:17PM +0200, Alexandre Chartre wrote:
> 
> On 5/5/20 3:53 PM, Thomas Gleixner wrote:
> > The pagefault handler cannot use the regular idtentry_enter() because on
> > that invokes rcu_irq_enter() the pagefault was caused in the kernel.
> 
> I am struggling to understand this part of the sentence: "because on
> that invokes rcu_irq_enter() the pagefault was caused in the kernel."
> 
> Do you mean: "because that invokes rcu_irq_enter() if the pagefault was
> caused in the kernel." ?

The problem is that rcu_irq_enter() uses per-cpu state and expect
rcu_irq_exit() to be called on the very same cpu we did enter on.

However, #PF likes to schedule and breaks that expectation.

While there are more exceptions that schedule when from userspace, #PF
is the only one that does so when from kernel space, which makes is
'special'.