Re: [PATCH v2] mm: workingset: replace IRQ-off check with a lockdep assert.

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Feb 11, 2019 at 12:38:29PM +0100, Sebastian Andrzej Siewior wrote:
> Commit
> 
>   68d48e6a2df57 ("mm: workingset: add vmstat counter for shadow nodes")
> 
> introduced an IRQ-off check to ensure that a lock is held which also
> disabled interrupts. This does not work the same way on -RT because none
> of the locks, that are held, disable interrupts.
> Replace this check with a lockdep assert which ensures that the lock is
> held.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

I'm not against checking for the lock, but if IRQs aren't disabled,
what ensures __mod_lruvec_state() is safe? I'm guessing it's because
preemption is disabled and irq handlers are punted to process context.

That said, it seems weird to me that

	spin_lock_irqsave();
	BUG_ON(!irqs_disabled());
	spin_unlock_irqrestore();

would trigger. Wouldn't it make sense to have a raw_irqs_disabled() or
something and keep the irqs_disabled() abstraction layer intact?