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

[Peter Zijlstra]

On Tue, Jun 06, 2017 at 10:50:48AM -0700, Paul E. McKenney wrote:
> 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.

Oh, duh, so much for being able to read...