Re: [PATCH v2 05/12] x86: rework arch_local_irq_restore() to not use popf

From: Jürgen Groß
Date: Mon Nov 23 2020 - 00:23:31 EST


On 22.11.20 22:44, Andy Lutomirski wrote:
On Sat, Nov 21, 2020 at 10:55 PM Jürgen Groß <jgross@xxxxxxxx> wrote:

On 20.11.20 12:59, Peter Zijlstra wrote:
On Fri, Nov 20, 2020 at 12:46:23PM +0100, Juergen Gross wrote:
+static __always_inline void arch_local_irq_restore(unsigned long flags)
+{
+ if (!arch_irqs_disabled_flags(flags))
+ arch_local_irq_enable();
+}

If someone were to write horrible code like:

local_irq_disable();
local_irq_save(flags);
local_irq_enable();
local_irq_restore(flags);

we'd be up some creek without a paddle... now I don't _think_ we have
genius code like that, but I'd feel saver if we can haz an assertion in
there somewhere...

Maybe something like:

#ifdef CONFIG_DEBUG_ENTRY // for lack of something saner
WARN_ON_ONCE((arch_local_save_flags() ^ flags) & X86_EFLAGS_IF);
#endif

At the end?

I'd like to, but using WARN_ON_ONCE() in include/asm/irqflags.h sounds
like a perfect receipt for include dependency hell.

We could use a plain asm("ud2") instead.

How about out-of-lining it:

#ifdef CONFIG_DEBUG_ENTRY
extern void warn_bogus_irqrestore();
#endif

static __always_inline void arch_local_irq_restore(unsigned long flags)
{
if (!arch_irqs_disabled_flags(flags)) {
arch_local_irq_enable();
} else {
#ifdef CONFIG_DEBUG_ENTRY
if (unlikely(arch_local_irq_save() & X86_EFLAGS_IF))
warn_bogus_irqrestore();
#endif
}


This couldn't be a WARN_ON_ONCE() then (or it would be a catch all).
Another approach might be to open-code the WARN_ON_ONCE(), like:

#ifdef CONFIG_DEBUG_ENTRY
extern void warn_bogus_irqrestore(bool *once);
#endif

static __always_inline void arch_local_irq_restore(unsigned long flags)
{
if (!arch_irqs_disabled_flags(flags))
arch_local_irq_enable();
#ifdef CONFIG_DEBUG_ENTRY
{
static bool once;

if (unlikely(arch_local_irq_save() & X86_EFLAGS_IF))
warn_bogus_irqrestore(&once);
}
#endif
}


Juergen

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