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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index 36e1f82faed1..681bf6bc04a5 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -35,8 +35,8 @@
>  
>  static int init_srcu_struct_fields(struct srcu_struct *sp)
>  {
> -	sp->srcu_lock_nesting[0] = 0;
> -	sp->srcu_lock_nesting[1] = 0;
> +	atomic_set(&sp->srcu_lock_nesting[0], 0);
> +	atomic_set(&sp->srcu_lock_nesting[1], 0);
>  	init_swait_queue_head(&sp->srcu_wq);
>  	sp->srcu_gp_seq = 0;
>  	rcu_segcblist_init(&sp->srcu_cblist);
> @@ -86,7 +86,8 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
>   */
>  void cleanup_srcu_struct(struct srcu_struct *sp)
>  {
> -	WARN_ON(sp->srcu_lock_nesting[0] || sp->srcu_lock_nesting[1]);
> +	WARN_ON(atomic_read(&sp->srcu_lock_nesting[0]) ||
> +		atomic_read(&sp->srcu_lock_nesting[1]));
>  	flush_work(&sp->srcu_work);
>  	WARN_ON(rcu_seq_state(sp->srcu_gp_seq));
>  	WARN_ON(sp->srcu_gp_running);
> @@ -97,7 +98,7 @@ 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)
> @@ -105,21 +106,19 @@ int __srcu_read_lock(struct srcu_struct *sp)
>  	int idx;
>  
>  	idx = READ_ONCE(sp->srcu_idx);
> -	WRITE_ONCE(sp->srcu_lock_nesting[idx], sp->srcu_lock_nesting[idx] + 1);
> +	atomic_inc(&sp->srcu_lock_nesting[idx]);
>  	return idx;
>  }
>  EXPORT_SYMBOL_GPL(__srcu_read_lock);
>  
>  /*
>   * Removes the count for the old reader from the appropriate element of
> - * the srcu_struct.  Must be called from process context.
> + * the srcu_struct.
>   */
>  void __srcu_read_unlock(struct srcu_struct *sp, int idx)
>  {
> -	int newval = sp->srcu_lock_nesting[idx] - 1;
> -
> -	WRITE_ONCE(sp->srcu_lock_nesting[idx], newval);
> -	if (!newval && READ_ONCE(sp->srcu_gp_waiting))
> +	if (atomic_dec_return_relaxed(&sp->srcu_lock_nesting[idx]) == 0 &&
> +	    READ_ONCE(sp->srcu_gp_waiting))
>  		swake_up(&sp->srcu_wq);
>  }
>  EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> @@ -148,7 +147,7 @@ void srcu_drive_gp(struct work_struct *wp)
>  	idx = sp->srcu_idx;
>  	WRITE_ONCE(sp->srcu_idx, !sp->srcu_idx);
>  	WRITE_ONCE(sp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
> -	swait_event(sp->srcu_wq, !READ_ONCE(sp->srcu_lock_nesting[idx]));
> +	swait_event(sp->srcu_wq, !atomic_read(&sp->srcu_lock_nesting[idx]));
>  	WRITE_ONCE(sp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
>  	rcu_seq_end(&sp->srcu_gp_seq);

I'm not entirely sure this is actually needed. TINY_SRCU is !PREEMPT &&
!SMP. So that means all we need is to be safe from IRQs.

Now, do we (want) support things like:

<IRQ>
  srcu_read_lock();
</IRQ>

  srcu_read_lock();

  srcu_read_unlock();

<IRQ>
  srcu_read_unlock();
</IRC>


_OR_

do we already (or want to) mandate that SRCU usage in IRQs must be
balanced? That is, if it is used from IRQ context it must do an equal
amount of srcu_read_lock() and srcu_read_unlock()s?

Because if we have the balance requirement (as we do for
preempt_disable()) then even on load-store architectures the current
code should be sufficient (since if an interrupt does as many dec's as
it does inc's, the actual value will not change over an interrupt, and
our load from before the interrupt is still valid).