Messages in this thread | | | Date | Mon, 16 Jan 2017 03:22:39 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 1/6] rcu: Abstract the dynticks momentary-idle operation |
| |
On Sun, Jan 15, 2017 at 11:39:51PM -0800, Josh Triplett wrote: > On Sat, Jan 14, 2017 at 12:54:40AM -0800, Paul E. McKenney wrote: > > This commit is the first step towards full abstraction of all accesses to > > the ->dynticks counter, implementing the previously open-coded atomic add > > of two in a new rcu_dynticks_momentary_idle() function. This abstraction > > will ease changes to the ->dynticks counter operation. > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > This change has an additional effect not documented in the commit > message: it eliminates the smp_mb__before_atomic and > smp_mb__after_atomic calls. Can you please document that in the commit > message, and explain why that doesn't cause a problem?
The trick is that the old code used the non-value-returning atomic_add(), which does not imply ordering, hence the smp_mb__before_atomic() and smp_mb__after_atomic() calls. The new code uses atomic_add_return(), which does return a value, and therefore implies full ordering in and of itself.
How would you like me to proceed?
Thanx, Paul
> > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -281,6 +281,19 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = { > > #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ > > }; > > > > +/* > > + * Do a double-increment of the ->dynticks counter to emulate a > > + * momentary idle-CPU quiescent state. > > + */ > > +static void rcu_dynticks_momentary_idle(void) > > +{ > > + struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > > + int special = atomic_add_return(2, &rdtp->dynticks); > > + > > + /* It is illegal to call this from idle state. */ > > + WARN_ON_ONCE(!(special & 0x1)); > > +} > > + > > DEFINE_PER_CPU_SHARED_ALIGNED(unsigned long, rcu_qs_ctr); > > EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr); > > > > @@ -300,7 +313,6 @@ EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr); > > static void rcu_momentary_dyntick_idle(void) > > { > > struct rcu_data *rdp; > > - struct rcu_dynticks *rdtp; > > int resched_mask; > > struct rcu_state *rsp; > > > > @@ -327,10 +339,7 @@ static void rcu_momentary_dyntick_idle(void) > > * quiescent state, with no need for this CPU to do anything > > * further. > > */ > > - rdtp = this_cpu_ptr(&rcu_dynticks); > > - smp_mb__before_atomic(); /* Earlier stuff before QS. */ > > - atomic_add(2, &rdtp->dynticks); /* QS. */ > > - smp_mb__after_atomic(); /* Later stuff after QS. */ > > + rcu_dynticks_momentary_idle(); > > break; > > } > > } > > -- > > 2.5.2 > > >
| |