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

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

On Tue, Jun 06, 2017 at 07:23:42PM +0200, Peter Zijlstra wrote:
> 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?

Unlike Tiny SRCU, Classic and Tree SRCU increment one counter
(->srcu_lock_count[]) and decrement another (->srcu_unlock_count[]).
So balanced srcu_read_lock() and srcu_read_unlock() within an irq
handler would increment both counters, with no decrements.  Therefore,
__srcu_read_lock()'s counter manipulation needs to be irq-safe.

							Thanx, Paul