Re: BUG or not? GFP_KERNEL with interrupts disabled.

On Thu, 27 Mar 2003, David S. Miller wrote:

   Further more, holding a lock_irq doesn't mean bottom halves are disabled
   too, it just means interrupts are disabled and no *new* softirq can be
   queued. Consider the following situation:
   
I think local_bh_enable() should check irqs_disabled() and honour that.
What you are showing here, that BH's can run via local_bh_enable()
even when IRQs are disabled, is a BUG().

I'd disagree.

I do agree that we should obviously not run bottom halves with interrupts 
disabled, but I think the _real_ bug is doing "local_bh_enable()" in the 
first place. It's a nesting bug: you must nest the "stronger" lock inside 
the weaker one, which means that the following is right:

	local_bh_disable()
		..
		local_irq_disable()
		...
		local_irq_enable()
		..
	local_bh_enable()

and this is WRONG:

	local_irq_disable() (or spinlock)
		..
		local_bh_disable()
		..
		local_bh_enable()	!BUG BUG BUG!
		..
	local_irq_enable()

So the bug is, in my opinion, not in BK handling, but in the caller.

I missed the start of this thread, so I don't know how hard this is to 
fix. But if you have a buggy sequence, the _simple_ fix may be to do 
somehting like this:

+++	local_bh_disable()
	local_irq_disable() (or spinlock)
		..
		local_bh_disable()
		..
		local_bh_enable()	! now it's a no-op and no longer a bug
		..
	local_irq_enable()
+++	local_bh_enable()

What's the code sequence?

		Linus