Subject: Re: SMP API things, lock debugging, etc.
To: Stefan Grefen <>
From: Jason Thorpe <>
List: tech-smp
Date: 07/27/1999 15:00:47
On Tue, 27 Jul 1999 23:27:08 +0200 
 Stefan Grefen <> wrote:

 > OK lets define the (S)MP semantics of spl*().

Okay!  :-)

 > Do we block the interrupts on the affected processor or on all processors?

It blocks the specified interrupts on the "current processor", not all.

 > Only if it blocks interrupts on all processors this method prevents 
 > deadlocks.

How so?

	CPU 0: s = splfoo(), simple_lock(foo_lock)
	CPU 1: foointr (implicit splfoo), simple_lock(foo_lock) ... spins
	CPU 0: finishes with foo, simple_unlock(foo_lock), splx(s)
	CPU 1: acquires foo_lock, does its thing, simple_unlock(foo_lock)

Can you describe to me a situation where failing to block interrupts on
all processors will cause deadlock (cases where one wasn't careful in
designing the locking protocol for the given subsystem don't count :-)

        -- Jason R. Thorpe <>