linux-kernel - Re: [patch V2 07/17] x86/entry/64: Remove redundant interrupt disable

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Message-ID: <alpine.DEB.2.21.1910240146200.1852@nanos.tec.linutronix.de>
Date:   Thu, 24 Oct 2019 01:52:33 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-arch@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [patch V2 07/17] x86/entry/64: Remove redundant interrupt
 disable

On Wed, 23 Oct 2019, Josh Poimboeuf wrote:

> On Wed, Oct 23, 2019 at 02:27:12PM +0200, Thomas Gleixner wrote:
> > Now that the trap handlers return with interrupts disabled, the
> > unconditional disabling of interrupts in the low level entry code can be
> > removed along with the trace calls.
> > 
> > Add debug checks where appropriate.
> 
> This seems a little scary.  Does anybody other than Andy actually run
> with CONFIG_DEBUG_ENTRY?

I do.

> What happens if somebody accidentally leaves irqs enabled?  How do we
> know you found all the leaks?

For the DO_ERROR() ones that's trivial:

 #define DO_ERROR(trapnr, signr, sicode, addr, str, name)                  \
 dotraplinkage void do_##name(struct pt_regs *regs, long error_code)	   \
 {									   \
 	do_error_trap(regs, error_code, str, trapnr, signr, sicode, addr); \
+	lockdep_assert_irqs_disabled();					   \
 }
 
 DO_ERROR(X86_TRAP_DE,     SIGFPE,  FPE_INTDIV,   IP, "divide error",        divide_error)

Now for the rest we surely could do:

dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
{
	__do_bounds(regs, error_code);
	lockdep_assert_irqs_disabled();
}

and move the existing body into a static function so independent of any
(future) return path there the lockdep assert will be invoked.

Thanks,

	tglx