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

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Date:	Wed, 15 Jul 2015 01:26:21 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	luto@...nel.org, mingo@...nel.org, luto@...capital.net,
	keescook@...omium.org, torvalds@...ux-foundation.org,
	peterz@...radead.org, paulmck@...ux.vnet.ibm.com, oleg@...hat.com,
	hpa@...or.com, linux-kernel@...r.kernel.org,
	vda.linux@...glemail.com, riel@...hat.com, bp@...en8.de,
	tglx@...utronix.de, dvlasenk@...hat.com, brgerst@...il.com
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] x86/irq, context_tracking: Document how IRQ
 context tracking works and add an RCU assertion

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@...nel.org>
> AuthorDate: Fri, 3 Jul 2015 12:44:34 -0700
> Committer:  Ingo Molnar <mingo@...nel.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@...nel.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: Denys Vlasenko <vda.linux@...glemail.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: paulmck@...ux.vnet.ibm.com
> Link: http://lkml.kernel.org/r/e8bdc4ed0193fb2fd130f3d6b7b8023e2ec1ab62.1435952415.git.luto@kernel.org
> Signed-off-by: Ingo Molnar <mingo@...nel.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.
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