linux-kernel - [PATCH] [RFC] CFQ: simplify radix tree lookup in cfq_cic

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Date:	Wed, 03 Aug 2011 22:14:32 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] [RFC] CFQ: simplify radix tree lookup in cfq_cic_lookup()

0) Not tested and not signed-off.

1) This is to see whether I understand the cfqd->cic_index usage (at
least, part of it).

2) If the lookup of a cic in the radix tree turned up a "dead" cic, then
that cic will be dropped. There's no reason to again try to lookup that
cic: that lookup should return NULL. (If it doesn't return NULL, we seem
to be in trouble.) So there's no need for a do {[...]} while (1) loop
and this code can be simplified a little.

3) Does this make sense?


Paul Bolle
---
 block/cfq-iosched.c |   28 +++++++++++++---------------
 1 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 1f96ad6..0d33d8c 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3120,22 +3120,20 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
 		return cic;
 	}
 
-	do {
-		cic = radix_tree_lookup(&ioc->radix_root, cfqd->cic_index);
-		rcu_read_unlock();
-		if (!cic)
-			break;
-		if (unlikely(cic->key != cfqd)) {
-			cfq_drop_dead_cic(cfqd, ioc, cic);
-			rcu_read_lock();
-			continue;
-		}
+	cic = radix_tree_lookup(&ioc->radix_root, cfqd->cic_index);
+	rcu_read_unlock();
 
-		spin_lock_irqsave(&ioc->lock, flags);
-		rcu_assign_pointer(ioc->ioc_data, cic);
-		spin_unlock_irqrestore(&ioc->lock, flags);
-		break;
-	} while (1);
+	if (!cic)
+		return NULL;
+
+	if (unlikely(cic->key != cfqd)) {
+		cfq_drop_dead_cic(cfqd, ioc, cic);
+		return NULL;
+	}
+
+	spin_lock_irqsave(&ioc->lock, flags);
+	rcu_assign_pointer(ioc->ioc_data, cic);
+	spin_unlock_irqrestore(&ioc->lock, flags);
 
 	return cic;
 }
-- 
1.7.6

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