Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

[Eric Dumazet]

On Mon, 2014-06-02 at 15:08 -0700, Paul E. McKenney wrote:

> diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
> index c639556f3fa0..c0120279dead 100644
> --- a/kernel/rcu/srcu.c
> +++ b/kernel/rcu/srcu.c
> @@ -295,12 +295,15 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
>  int __srcu_read_lock(struct srcu_struct *sp)
>  {
>       int idx;
> +     unsigned long *lp;
>  
>       idx = ACCESS_ONCE(sp->completed) & 0x1;
>       preempt_disable();
> -     ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
> +     lp = this_cpu_ptr(&sp->per_cpu_ref->c[idx]);
> +     ACCESS_ONCE(*lp) = *lp + 1;
>       smp_mb(); /* B */  /* Avoid leaking the critical section. */
> -     ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
> +     lp = this_cpu_ptr(&sp->per_cpu_ref->seq[idx]);
> +     ACCESS_ONCE(*lp) = *lp + 1;
>       preempt_enable();
>       return idx;
>  

This probably could use the following 

diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index c639556f3fa0..3a97eb6f9076 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -298,9 +298,9 @@ int __srcu_read_lock(struct srcu_struct *sp)
 
        idx = ACCESS_ONCE(sp->completed) & 0x1;
        preempt_disable();
-       ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
+       this_cpu_inc(sp->per_cpu_ref->c[idx]);
        smp_mb(); /* B */  /* Avoid leaking the critical section. */
-       ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
+       this_cpu_inc(sp->per_cpu_ref->seq[idx]);
        preempt_enable();
        return idx;
 }


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