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

Eric W. Biederman
2018-10-22T17:31:04Z

"Paul E. McKenney" <paulmck@linux.ibm.com> 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