Re: [PATCH tip/core/rcu 06/17] rcu: Eliminate read-modify-write ACCESS_ONCE() calls

[Pranith Kumar]

Hi Paul,

On Mon, Jul 7, 2014 at 6:38 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
>
> RCU contains code of the following forms:
>
>         ACCESS_ONCE(x)++;
>         ACCESS_ONCE(x) += y;
>         ACCESS_ONCE(x) -= y;
>
> Now these constructs do operate correctly, but they really result in a
> pair of volatile accesses, one to do the load and another to do the store.
> This can be confusing, as the casual reader might well assume that (for
> example) gcc might generate a memory-to-memory add instruction for each
> of these three cases.  In fact, gcc will do no such thing.  Also, there
> is a good chance that the kernel will move to separate load and store
> variants of ACCESS_ONCE(), and constructs like the above could easily
> confuse both people and scripts attempting to make that sort of change.
> Finally, most of RCU's read-modify-write uses of ACCESS_ONCE() really
> only need the store to be volatile, so that the read-modify-write form
> might be misleading.
>
> This commit therefore changes the above forms in RCU so that each instance
> of ACCESS_ONCE() either does a load or a store, but not both.  In a few
> cases, ACCESS_ONCE() was not critical, for example, for maintaining
> statisitics.  In these cases, ACCESS_ONCE() has been dispensed with
> entirely.
>

Is there any reason why |=, &= cannot be replaced similarly? Also
there are a few more in tree_plugin.h. Please find patch below:

Signed-off-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
---
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dac6d20..f500395 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1700,7 +1700,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int
fqs_state_in)
        if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
                raw_spin_lock_irq(&rnp->lock);
                smp_mb__after_unlock_lock();
-               ACCESS_ONCE(rsp->gp_flags) &= ~RCU_GP_FLAG_FQS;
+               ACCESS_ONCE(rsp->gp_flags) = rsp->gp_flags & ~RCU_GP_FLAG_FQS;
                raw_spin_unlock_irq(&rnp->lock);
        }
        return fqs_state;
@@ -2514,7 +2514,7 @@ static void force_quiescent_state(struct rcu_state *rsp)
                raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
                return;  /* Someone beat us to it. */
        }
-       ACCESS_ONCE(rsp->gp_flags) |= RCU_GP_FLAG_FQS;
+       ACCESS_ONCE(rsp->gp_flags) = rsp->gp_flags | RCU_GP_FLAG_FQS;
        raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
        wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
 }
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1a4ab26..752d382 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -897,7 +897,8 @@ void synchronize_rcu_expedited(void)

        /* Clean up and exit. */
        smp_mb(); /* ensure expedited GP seen before counter increment. */
-       ACCESS_ONCE(sync_rcu_preempt_exp_count)++;
+       ACCESS_ONCE(sync_rcu_preempt_exp_count) =
+                                       sync_rcu_preempt_exp_count + 1;
 unlock_mb_ret:
        mutex_unlock(&sync_rcu_preempt_exp_mutex);
 mb_ret:
@@ -2307,8 +2308,9 @@ static int rcu_nocb_kthread(void *arg)
                        list = next;
                }
                trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
-               ACCESS_ONCE(rdp->nocb_p_count) -= c;
-               ACCESS_ONCE(rdp->nocb_p_count_lazy) -= cl;
+               ACCESS_ONCE(rdp->nocb_p_count) = rdp->nocb_p_count - c;
+               ACCESS_ONCE(rdp->nocb_p_count_lazy) =
+                                               rdp->nocb_p_count_lazy - cl;
                rdp->n_nocbs_invoked += c;
        }
        return 0;

-- 
Pranith
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