Re: Interrupts, smp_load_acquire(), smp_store_release(), etc.

[Alan Stern <stern@rowland.harvard.edu>]

On Sat, 20 Oct 2018, Paul E. McKenney wrote:

> The second (informal) litmus test has a more interesting Linux-kernel
> counterpart:
> 
> 	void t1_interrupt(void)
> 	{
> 		r0 = READ_ONCE(y);
> 		smp_store_release(&x, 1);
> 	}
> 
> 	void t1(void)
> 	{
> 		smp_store_release(&y, 1);
> 	}
> 
> 	void t2(void)
> 	{
> 		r1 = smp_load_acquire(&x);
> 		r2 = smp_load_acquire(&y);
> 	}
> 
> On store-reordering architectures that implement smp_store_release()
> as a memory-barrier instruction followed by a store, the interrupt could
> arrive betweentimes in t1(), so that there would be no ordering between
> t1_interrupt()'s store to x and t1()'s store to y.  This could (again,
> in paranoid theory) result in the outcome r0==0 && r1==0 && r2==1.

This is disconcerting only if we assume that t1_interrupt() has to be
executed by the same CPU as t1().  If the interrupt could be fielded by
a different CPU then the paranoid outcome is perfectly understandable,
even in an SC context.

So the question really should be limited to situations where a handler 
is forced to execute in the context of a particular thread.  While 
POSIX does allow such restrictions for user programs, I'm not aware of 
any similar mechanism in the kernel.

Alan