linux-kernel - Re: [PATCH 07/14] x86: Use lockdep to assert IRQs are disabled/enabled

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Message-ID: <20171022092025.GJ6524@worktop.programming.kicks-ass.net>
Date:   Sun, 22 Oct 2017 11:20:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 07/14] x86: Use lockdep to assert IRQs are
 disabled/enabled

On Fri, Oct 20, 2017 at 02:56:04AM +0200, Frederic Weisbecker wrote:
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 03505ff..b4f3a55 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -186,7 +186,7 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
>  
>  	addr_limit_user_check();
>  
> -	if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
> +	if (!lockdep_assert_irqs_disabled())
>  		local_irq_disable();
>  
>  	lockdep_sys_exit();

So this is the only site that ever uses the return value; and for this
you've chosen the wrong value for !CONFIG_PROVE_LOCKING (namely 1),
resulting in an unconditional CLI here for !lockdep kernels.

How about we replace that whole thing with a simple:

	lockdep_assert_irqs_disabled();

And leave it at that, allowing us to remove the return value thing
entirely.

The whole if !disabled, disable logic is uber paranoid programming, but
I don't think we've ever seen that WARN trigger, and if it does (and
then burns the kernel) we at least know wtf happend.