Re: [tip:x86/asm] x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion

Frederic Weisbecker
2015-07-14T23:26:26Z

On Tue, Jul 07, 2015 at 03:54:32AM -0700, tip-bot for Andy Lutomirski wrote:
> Commit-ID: 0333a209cbf600e980fc55c24878a56f25f48b65
> Gitweb: http://git.kernel.org/tip/0333a209cbf600e980fc55c24878a56f25f48b65
> Author: Andy Lutomirski <luto@kernel.org>
> AuthorDate: Fri, 3 Jul 2015 12:44:34 -0700
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 7 Jul 2015 10:59:10 +0200
>
> x86/irq, context_tracking: Document how IRQ context tracking works and add an RCU assertion
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Denys Vlasenko <vda.linux@googlemail.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: paulmck@linux.vnet.ibm.com
> Link: http://lkml.kernel.org/r/e8bdc4ed0193fb2fd130f3d6b7b8023e2ec1ab62.1435952415.git.luto@kernel.org
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> arch/x86/kernel/irq.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> index 88b36648..6233de0 100644
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -216,8 +216,23 @@ __visible unsigned int __irq_entry do_IRQ(struct pt_regs *regs)
> unsigned vector = ~regs->orig_ax;
> unsigned irq;
>
> + /*
> + * NB: Unlike exception entries, IRQ entries do not reliably
> + * handle context tracking in the low-level entry code. This is
> + * because syscall entries execute briefly with IRQs on before
> + * updating context tracking state, so we can take an IRQ from
> + * kernel mode with CONTEXT_USER. The low-level entry code only
> + * updates the context if we came from user mode, so we won't
> + * switch to CONTEXT_KERNEL. We'll fix that once the syscall
> + * code is cleaned up enough that we can cleanly defer enabling
> + * IRQs.
> + */
> +

Now is it a problem to take interrupts in kernel mode with CONTEXT_USER?
I'm not sure it's worth trying to make it not happen.

> entering_irq();
>
> + /* entering_irq() tells RCU that we're not quiescent. Check it. */
> + rcu_lockdep_assert(rcu_is_watching(), "IRQ failed to wake up RCU");

Why do we need to check that?

> +
> irq = __this_cpu_read(vector_irq[vector]);
>
> if (!handle_irq(irq, regs)) {

Thanks.