On Sun, Nov 22, 2020 at 01:44:53PM -0800, Andy Lutomirski wrote:

On Sat, Nov 21, 2020 at 10:55 PM Jürgen Groß [off-list ref] wrote:

quoted

On 20.11.20 12:59, Peter Zijlstra wrote:

quoted

On Fri, Nov 20, 2020 at 12:46:23PM +0100, Juergen Gross wrote:

quoted

+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
}

I was just talking to Peter on IRC about implementing the same thing for
arm64, so could we put this in the generic irqflags code? IIUC we can
use raw_irqs_disabled() to do the check.

As this isn't really entry specific (and IIUC the cases this should
catch would break lockdep today), maybe we should add a new
DEBUG_IRQFLAGS for this, that DEBUG_LOCKDEP can also select?

Something like:

#define local_irq_restore(flags)                               \
       do {                                                    \
               if (!raw_irqs_disabled_flags(flags)) {          \
                       trace_hardirqs_on();                    \
               } else if (IS_ENABLED(CONFIG_DEBUG_IRQFLAGS) {  \
                       if (unlikely(raw_irqs_disabled())       \
                               warn_bogus_irqrestore();        \
               }                                               \
               raw_local_irq_restore(flags);                   \
        } while (0)

... perhaps? (ignoring however we deal with once-ness).

Thanks,
Mark.
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