linux-kernel - Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

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Date:   Tue, 6 Jun 2017 19:23:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, Paolo Bonzini <pbonzini@...hat.com>,
        kvm@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU
 from both process and interrupt context

On Mon, Jun 05, 2017 at 03:09:50PM -0700, Paul E. McKenney wrote:
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 3ae8474557df..157654fa436a 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -357,7 +357,7 @@ EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
>  
>  /*
>   * Counts the new reader in the appropriate per-CPU element of the
> - * srcu_struct.  Must be called from process context.
> + * srcu_struct.
>   * Returns an index that must be passed to the matching srcu_read_unlock().
>   */
>  int __srcu_read_lock(struct srcu_struct *sp)
> @@ -365,7 +365,7 @@ int __srcu_read_lock(struct srcu_struct *sp)
>  	int idx;
>  
>  	idx = READ_ONCE(sp->srcu_idx) & 0x1;
> -	__this_cpu_inc(sp->sda->srcu_lock_count[idx]);
> +	this_cpu_inc(sp->sda->srcu_lock_count[idx]);
>  	smp_mb(); /* B */  /* Avoid leaking the critical section. */
>  	return idx;
>  }

So again, the change is to make this an IRQ safe operation, however if
we have this balance requirement, the IRQ will not visibly change the
value and load-store should be good again, no?

Or am I missing some other detail with this implementation?