Re: smp_mb__after_spinlock requirement too strong?

[Boqun Feng <boqun.feng@gmail.com>]

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On Sun, Mar 11, 2018 at 03:55:41PM +0800, 焦晓冬 wrote:
> Peter pointed out in this patch https://patchwork.kernel.org/patch/9771921/
> that the spinning-lock used at __schedule() should be RCsc to ensure
> visibility of writes prior to __schedule when the task is to be migrated to
> another CPU.
> 
> And this is emphasized at the comment of the newly introduced
> smp_mb__after_spinlock(),
> 
>  * This barrier must provide two things:
>  *
>  *   - it must guarantee a STORE before the spin_lock() is ordered against a
>  *     LOAD after it, see the comments at its two usage sites.
>  *
>  *   - it must ensure the critical section is RCsc.
>  *
>  * The latter is important for cases where we observe values written by other
>  * CPUs in spin-loops, without barriers, while being subject to scheduling.
>  *
>  * CPU0         CPU1            CPU2
>  *
>  *          for (;;) {
>  *            if (READ_ONCE(X))
>  *              break;
>  *          }
>  * X=1
>  *          <sched-out>
>  *                      <sched-in>
>  *                      r = X;
>  *
>  * without transitivity it could be that CPU1 observes X!=0 breaks the loop,
>  * we get migrated and CPU2 sees X==0.
> 
> which is used at,
> 
> __schedule(bool preempt) {
>     ...
>     rq_lock(rq, &rf);
>     smp_mb__after_spinlock();
>     ...
> }
> .
> 
> If I didn't miss something, I found this kind of visibility is __not__
> necessarily
> depends on the spinning-lock at __schedule being RCsc.
> 
> In fact, as for runnable task A, the migration would be,
> 
>  CPU0         CPU1            CPU2
> 
> <ACCESS before schedule out A>
> 
> lock(rq0)
> schedule out A
> unock(rq0)
> 
>               lock(rq0)
>               remove A from rq0
>               unlock(rq0)
> 
>               lock(rq2)
>               add A into rq2
>               unlock(rq2)
>                                         lock(rq2)
>                                         schedule in A
>                                         unlock(rq2)
> 
>                                         <ACCESS after schedule in A>
> 
> <ACCESS before schedule out A> happens-before
> unlock(rq0) happends-before
> lock(rq0) happends-before
> unlock(rq2) happens-before
> lock(rq2) happens-before
> <ACCESS after schedule in A>
> 

But without RCsc lock, you cannot guarantee that a write propagates to
CPU 0 and CPU 2 at the same time, so the same write may propagate to
CPU0 before <ACCESS before schedule out A> but propagate to CPU 2 after
<ACCESS after scheduler in A>. So..

Regards,
Boqun

> And for stopped tasks,
> 
>  CPU0         CPU1            CPU2
> 
> <ACCESS before schedule out A>
> 
> lock(rq0)
> schedule out A
> remove A from rq0
> store-release(A->on_cpu)
> unock(rq0)
> 
>               load_acquire(A->on_cpu)
>               set_task_cpu(A, 2)
> 
>               lock(rq2)
>               add A into rq2
>               unlock(rq2)
> 
>                                         lock(rq2)
>                                         schedule in A
>                                         unlock(rq2)
> 
>                                         <ACCESS after schedule in A>
> 
> <ACCESS before schedule out A> happens-before
> store-release(A->on_cpu)  happens-before
> load_acquire(A->on_cpu)  happens-before
> unlock(rq2) happens-before
> lock(rq2) happens-before
> <ACCESS after schedule in A>
> 
> So, I think the only requirement to smp_mb__after_spinlock is
> to guarantee a STORE before the spin_lock() is ordered
> against a LOAD after it. So we could remove the RCsc requirement
> to allow more efficient implementation.
> 
> Did I miss something or this RCsc requirement does not really matter?

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