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From:   Christoph Hellwig <hch@lst.de>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Minchan Kim <minchan@kernel.org>,
        Nitin Gupta <ngupta@vflare.org>, dm-devel@redhat.com,
        linux-block@vger.kernel.org, drbd-dev@lists.linbit.com,
        linux-bcache@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH 14/16] block: remove rcu_read_lock() from part_stat_lock()
Date:   Mon, 25 May 2020 13:30:12 +0200
Message-Id: <20200525113014.345997-15-hch@lst.de>
In-Reply-To: <20200525113014.345997-1-hch@lst.de>
References: <20200525113014.345997-1-hch@lst.de>
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From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

The RCU lock is required only in disk_map_sector_rcu() to lookup the
partition.  After that request holds reference to related hd_struct.

Replace get_cpu() with preempt_disable() - returned cpu index is unused.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
[hch: rebased]
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/genhd.c             | 11 ++++++++---
 include/linux/part_stat.h |  4 ++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 3e7df0a3e6bb0..1a76593276644 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -321,11 +321,12 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 	struct hd_struct *part;
 	int i;
 
+	rcu_read_lock();
 	ptbl = rcu_dereference(disk->part_tbl);
 
 	part = rcu_dereference(ptbl->last_lookup);
 	if (part && sector_in_part(part, sector) && hd_struct_try_get(part))
-		return part;
+		goto out_unlock;
 
 	for (i = 1; i < ptbl->len; i++) {
 		part = rcu_dereference(ptbl->part[i]);
@@ -339,10 +340,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 			if (!hd_struct_try_get(part))
 				break;
 			rcu_assign_pointer(ptbl->last_lookup, part);
-			return part;
+			goto out_unlock;
 		}
 	}
-	return &disk->part0;
+
+	part = &disk->part0;
+out_unlock:
+	rcu_read_unlock();
+	return part;
 }
 
 /**
diff --git a/include/linux/part_stat.h b/include/linux/part_stat.h
index 6644197980b92..a6b0938ce82e9 100644
--- a/include/linux/part_stat.h
+++ b/include/linux/part_stat.h
@@ -21,8 +21,8 @@ struct disk_stats {
  *
  * part_stat_read() can be called at any time.
  */
-#define part_stat_lock()	({ rcu_read_lock(); get_cpu(); })
-#define part_stat_unlock()	do { put_cpu(); rcu_read_unlock(); } while (0)
+#define part_stat_lock()	preempt_disable()
+#define part_stat_unlock()	preempt_enable()
 
 #define part_stat_get_cpu(part, field, cpu)				\
 	(per_cpu_ptr((part)->dkstats, (cpu))->field)
-- 
2.26.2