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 10:50:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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