[PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

[boy.wu]

From: Boy Wu <boy.wu at mediatek.com>

In 32bit SMP systems, if multiple CPUs call blkcg_print_stat,
it may cause blkcg_fill_root_iostats to have a concurrent problem
on the seqlock in u64_stats_update, which will cause a deadlock
on u64_stats_fetch_begin in blkcg_print_one_stat.

Thus, replace u64 sync with spinlock to protect iostat.

Fixes: ef45fe470e1e ("blk-cgroup: show global disk stats in root cgroup io.stat")
Signed-off-by: Boy Wu <boy.wu at mediatek.com>
---
Change in v2:
 - update commit message
 - Remove u64_sync
 - Replace spin_lock_irq with guard statement
 - Replace blkg->q->queue_lock with blkg_stat_lock
Change in v3:
 - update commit message
 - Add spinlock in blkg_iostat_set structure
 - Replace all u64_sync with spinlock for iostat
 - Replace blkg_stat_lock with iostat.spinlock
Change in v4:
 - update commit message
 - Remove spinlock in blkg_iostat_set structure
 - Replace iostat.spinlock with blkg_stat_lock
 - Add 32-bit systems only define
---
 block/blk-cgroup.c | 34 +++++++++++++---------------------
 1 file changed, 13 insertions(+), 21 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 37e6cc91d576..faa604c6fab9 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -329,7 +329,6 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
 	INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
 #endif
 
-	u64_stats_init(&blkg->iostat.sync);
 	for_each_possible_cpu(cpu) {
 		u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
 		per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;
@@ -632,18 +631,18 @@ static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
 static void __blkg_clear_stat(struct blkg_iostat_set *bis)
 {
 	struct blkg_iostat cur = {0};
-	unsigned long flags;
 
-	flags = u64_stats_update_begin_irqsave(&bis->sync);
 	blkg_iostat_set(&bis->cur, &cur);
 	blkg_iostat_set(&bis->last, &cur);
-	u64_stats_update_end_irqrestore(&bis->sync, flags);
 }
 
 static void blkg_clear_stat(struct blkcg_gq *blkg)
 {
 	int cpu;
 
+#if BITS_PER_LONG == 32
+	guard(raw_spinlock_irqsave)(&blkg_stat_lock);
+#endif
 	for_each_possible_cpu(cpu) {
 		struct blkg_iostat_set *s = per_cpu_ptr(blkg->iostat_cpu, cpu);
 
@@ -995,15 +994,12 @@ static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur,
 				struct blkg_iostat *last)
 {
 	struct blkg_iostat delta;
-	unsigned long flags;
 
 	/* propagate percpu delta to global */
-	flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
 	blkg_iostat_set(&delta, cur);
 	blkg_iostat_sub(&delta, last);
 	blkg_iostat_add(&blkg->iostat.cur, &delta);
 	blkg_iostat_add(last, &delta);
-	u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
 }
 
 static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
@@ -1034,7 +1030,6 @@ static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
 		struct blkcg_gq *blkg = bisc->blkg;
 		struct blkcg_gq *parent = blkg->parent;
 		struct blkg_iostat cur;
-		unsigned int seq;
 
 		/*
 		 * Order assignment of `next_bisc` from `bisc->lnode.next` in
@@ -1051,10 +1046,7 @@ static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
 			goto propagate_up; /* propagate up to parent only */
 
 		/* fetch the current per-cpu values */
-		do {
-			seq = u64_stats_fetch_begin(&bisc->sync);
-			blkg_iostat_set(&cur, &bisc->cur);
-		} while (u64_stats_fetch_retry(&bisc->sync, seq));
+		blkg_iostat_set(&cur, &bisc->cur);
 
 		blkcg_iostat_update(blkg, &cur, &bisc->last);
 
@@ -1112,7 +1104,6 @@ static void blkcg_fill_root_iostats(void)
 		struct blkcg_gq *blkg = bdev->bd_disk->queue->root_blkg;
 		struct blkg_iostat tmp;
 		int cpu;
-		unsigned long flags;
 
 		memset(&tmp, 0, sizeof(tmp));
 		for_each_possible_cpu(cpu) {
@@ -1133,10 +1124,10 @@ static void blkcg_fill_root_iostats(void)
 			tmp.bytes[BLKG_IOSTAT_DISCARD] +=
 				cpu_dkstats->sectors[STAT_DISCARD] << 9;
 		}
-
-		flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
+#if BITS_PER_LONG == 32
+		guard(raw_spinlock_irqsave)(&blkg_stat_lock);
+#endif
 		blkg_iostat_set(&blkg->iostat.cur, &tmp);
-		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
 	}
 }
 
@@ -1145,7 +1136,6 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
 	struct blkg_iostat_set *bis = &blkg->iostat;
 	u64 rbytes, wbytes, rios, wios, dbytes, dios;
 	const char *dname;
-	unsigned seq;
 	int i;
 
 	if (!blkg->online)
@@ -1157,16 +1147,18 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
 
 	seq_printf(s, "%s ", dname);
 
-	do {
-		seq = u64_stats_fetch_begin(&bis->sync);
-
+#if BITS_PER_LONG == 32
+	scoped_guard(raw_spinlock_irqsave, &blkg_stat_lock) {
+#endif
 		rbytes = bis->cur.bytes[BLKG_IOSTAT_READ];
 		wbytes = bis->cur.bytes[BLKG_IOSTAT_WRITE];
 		dbytes = bis->cur.bytes[BLKG_IOSTAT_DISCARD];
 		rios = bis->cur.ios[BLKG_IOSTAT_READ];
 		wios = bis->cur.ios[BLKG_IOSTAT_WRITE];
 		dios = bis->cur.ios[BLKG_IOSTAT_DISCARD];
-	} while (u64_stats_fetch_retry(&bis->sync, seq));
+#if BITS_PER_LONG == 32
+	}
+#endif
 
 	if (rbytes || wbytes || rios || wios) {
 		seq_printf(s, "rbytes=%llu wbytes=%llu rios=%llu wios=%llu dbytes=%llu dios=%llu",
-- 
2.18.0