Re: [PATCH 07/16] rcu: Save and restore irq flags in rcu_gp_cleanup()

Paul E. McKenney
2014-07-23T12:16:25Z

On Wed, Jul 23, 2014 at 01:09:44AM -0400, Pranith Kumar wrote:
> We use raw_spin_lock_irqsave/restore() family of functions throughout the code
> but for two locations. This commit replaces raw_spin_lock_irq()/unlock_irq()
> with irqsave/restore() in one such location. This is not strictly necessary,
> so I did not change the other location. I will update the other location if
> this is accepted :)
>
> This commit changes raw_spin_lock_irq()/unlock_irq() to lock_irqsave()/restore().
>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

I sympathize, as I used to take the approach that you are advocating.

The reason that I changed is that we -know- that interrupts are enabled
at this point in the code, so there is no point in incurring the extra
cognitive and machine overhead of the _irqsave() variant. Plus the
current code has documentation benefits -- it tells you that the author
felt that irqs could not possible be disabled here.

So sorry, but no.

Thanx, Paul

> ---
> kernel/rcu/tree.c | 14 +++++++-------
> 1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b14cecd..5dcbf36 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1706,13 +1706,13 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
> */
> static void rcu_gp_cleanup(struct rcu_state *rsp)
> {
> - unsigned long gp_duration;
> + unsigned long gp_duration, flags;
> bool needgp = false;
> int nocb = 0;
> struct rcu_data *rdp;
> struct rcu_node *rnp = rcu_get_root(rsp);
>
> - raw_spin_lock_irq(&rnp->lock);
> + raw_spin_lock_irqsave(&rnp->lock, flags);
> smp_mb__after_unlock_lock();
> gp_duration = jiffies - rsp->gp_start;
> if (gp_duration > rsp->gp_max)
> @@ -1726,7 +1726,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> * safe for us to drop the lock in order to mark the grace
> * period as completed in all of the rcu_node structures.
> */
> - raw_spin_unlock_irq(&rnp->lock);
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
>
> /*
> * Propagate new ->completed value to rcu_node structures so
> @@ -1738,7 +1738,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> * grace period is recorded in any of the rcu_node structures.
> */
> rcu_for_each_node_breadth_first(rsp, rnp) {
> - raw_spin_lock_irq(&rnp->lock);
> + raw_spin_lock_irqsave(&rnp->lock, flags);
> smp_mb__after_unlock_lock();
> ACCESS_ONCE(rnp->completed) = rsp->gpnum;
> rdp = this_cpu_ptr(rsp->rda);
> @@ -1746,11 +1746,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
> /* smp_mb() provided by prior unlock-lock pair. */
> nocb += rcu_future_gp_cleanup(rsp, rnp);
> - raw_spin_unlock_irq(&rnp->lock);
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> cond_resched();
> }
> rnp = rcu_get_root(rsp);
> - raw_spin_lock_irq(&rnp->lock);
> + raw_spin_lock_irqsave(&rnp->lock, flags);
> smp_mb__after_unlock_lock(); /* Order GP before ->completed update. */
> rcu_nocb_gp_set(rnp, nocb);
>
> @@ -1767,7 +1767,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> ACCESS_ONCE(rsp->gpnum),
> TPS("newreq"));
> }
> - raw_spin_unlock_irq(&rnp->lock);
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> }
>
> /*
> --
> 2.0.0.rc2
>