Hi Paul,
On Mon, Jul 7, 2014 at 6:38 PM, Paul E. McKenney
<paul...@linux.vnet.ibm.com> wrote:
> From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
>
> 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.pr...@gmail.com>
---
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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