linux-kernel - [PATCH 12/14] blk-cgroup: Make 64bit per cpu stats safe on 32bit arch

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Message-Id: <1305746006-5837-13-git-send-email-vgoyal@redhat.com>
Date:	Wed, 18 May 2011 15:13:24 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	linux-kernel@...r.kernel.org, jaxboe@...ionio.com
Cc:	dpshah@...gle.com, vgoyal@...hat.com
Subject: [PATCH 12/14] blk-cgroup: Make 64bit per cpu stats safe on 32bit arch

Some of the stats are 64bit and updation will be non atomic on 32bit
architecture. Use sequence counters on 32bit arch to make reading
of stats safe.

Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
---
 block/blk-cgroup.c |   27 ++++++++++++++++++++++-----
 block/blk-cgroup.h |    2 ++
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 34bfcef..3622518 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -400,14 +400,25 @@ void blkiocg_update_dispatch_stats(struct blkio_group *blkg,
 				uint64_t bytes, bool direction, bool sync)
 {
 	struct blkio_group_stats_cpu *stats_cpu;
+	unsigned long flags;
+
+	/*
+	 * Disabling interrupts to provide mutual exclusion between two
+	 * writes on same cpu. It probably is not needed for 64bit. Not
+	 * optimizing that case yet.
+	 */
+	local_irq_save(flags);
 
 	stats_cpu = this_cpu_ptr(blkg->stats_cpu);
 
+	u64_stats_update_begin(&stats_cpu->syncp);
 	stats_cpu->sectors += bytes >> 9;
 	blkio_add_stat(stats_cpu->stat_arr_cpu[BLKIO_STAT_CPU_SERVICED],
 			1, direction, sync);
 	blkio_add_stat(stats_cpu->stat_arr_cpu[BLKIO_STAT_CPU_SERVICE_BYTES],
 			bytes, direction, sync);
+	u64_stats_update_end(&stats_cpu->syncp);
+	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(blkiocg_update_dispatch_stats);
 
@@ -622,15 +633,21 @@ static uint64_t blkio_read_stat_cpu(struct blkio_group *blkg,
 {
 	int cpu;
 	struct blkio_group_stats_cpu *stats_cpu;
-	uint64_t val = 0;
+	u64 val = 0, tval;
 
 	for_each_possible_cpu(cpu) {
+		unsigned int start;
 		stats_cpu  = per_cpu_ptr(blkg->stats_cpu, cpu);
 
-		if (type == BLKIO_STAT_CPU_SECTORS)
-			val += stats_cpu->sectors;
-		else
-			val += stats_cpu->stat_arr_cpu[type][sub_type];
+		do {
+			start = u64_stats_fetch_begin(&stats_cpu->syncp);
+			if (type == BLKIO_STAT_CPU_SECTORS)
+				tval = stats_cpu->sectors;
+			else
+				tval = stats_cpu->stat_arr_cpu[type][sub_type];
+		} while(u64_stats_fetch_retry(&stats_cpu->syncp, start));
+
+		val += tval;
 	}
 
 	return val;
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index fd730a2..2622267 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -14,6 +14,7 @@
  */
 
 #include <linux/cgroup.h>
+#include <linux/u64_stats_sync.h>
 
 enum blkio_policy_id {
 	BLKIO_POLICY_PROP = 0,		/* Proportional Bandwidth division */
@@ -154,6 +155,7 @@ struct blkio_group_stats {
 struct blkio_group_stats_cpu {
 	uint64_t sectors;
 	uint64_t stat_arr_cpu[BLKIO_STAT_CPU_NR][BLKIO_STAT_TOTAL];
+	struct u64_stats_sync syncp;
 };
 
 struct blkio_group {
-- 
1.7.1

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