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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 11 May 2020 08:31:22 -0700
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Subject: Re: [patch V5 part 3 12/29] x86/entry/common: Provide idtentry_enter/exit()
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>
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On Mon, May 11, 2020 at 3:59 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Andy Lutomirski <luto@kernel.org> writes:
> >> + * Invoked by all exception/interrupt IDTENTRY handlers which are not
> >> + * returning through the paranoid exit path (all except NMI, #DF and the IST
> >> + * variants of #MC and #DB).
> >

> +void noinstr idtentry_exit(struct pt_regs *regs)
> +{
> +       lockdep_assert_irqs_disabled();
> +
> +       if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
> +               WARN_ON_ONCE(!on_thread_stack());

Whoops.  After sleeping on this, this is obviously wrong.  If this is
something like a page fault, we can be on an IST or IRQ stack.
Perhaps the actual condition should be:

WARN_ON_ONCE(!on_thread_stack() && (regs->flags & X86_FLAGS_IF) &&
preempt_count() == 0);

IOW, the actual condition we want is that, if the idtenter_entry/exit
code might schedule or if a cond_local_irq_enable() path might
schedule, we had better be on the correct stack.

Sorry for causing confusion.