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

[Eric W. Biederman]

"Paul E. McKenney" <paulmck@xxxxxxxxxxxxx> writes:

> On Sat, Oct 20, 2018 at 04:18:37PM -0400, Alan Stern wrote:
>> 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.

> Good point, and I was in fact assuming that t1() and t1_interrupt()
> were executing on the same CPU.
>
> This sort of thing happens naturally in the kernel when both t1()
> and t1_interrupt() are accessing per-CPU variables.

Interrupts have a cpumask of the cpus they may be dlievered on.

I believe networking does in fact have places where percpu actions
happen as well as interrupts pinned to a single cpu.  And yes I agree
percpu variables mean that you do not need to pin an interrupt to a
single cpu to cause this to happen.

Eric