linux-ext4 - [PATCH v2] ext4: Fix dead loop in ext4_mb_new

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Date:   Thu, 10 Sep 2020 11:08:06 +0800
From:   Ye Bin <yebin10@...wei.com>
To:     <tytso@....edu>, <adilger.kernel@...ger.ca>, <jack@...e.com>,
        <linux-ext4@...r.kernel.org>
CC:     Ye Bin <yebin10@...wei.com>
Subject: [PATCH v2] ext4: Fix dead loop in ext4_mb_new_blocks

As we test disk offline/online with running fsstress, we find fsstress
process is keeping running state.
kworker/u32:3-262   [004] ...1   140.787471: ext4_mb_discard_preallocations: dev 8,32 needed 114
....
kworker/u32:3-262   [004] ...1   140.787471: ext4_mb_discard_preallocations: dev 8,32 needed 114

ext4_mb_new_blocks
repeat:
	ext4_mb_discard_preallocations_should_retry(sb, ac, &seq)
		freed = ext4_mb_discard_preallocations
			ext4_mb_discard_group_preallocations
				this_cpu_inc(discard_pa_seq);
		---> freed == 0
		seq_retry = ext4_get_discard_pa_seq_sum
			for_each_possible_cpu(__cpu)
				__seq += per_cpu(discard_pa_seq, __cpu);
		if (seq_retry != *seq) {
			*seq = seq_retry;
			ret = true;
		}

As we see seq_retry is sum of discard_pa_seq every cpu, if
ext4_mb_discard_group_preallocations return zero discard_pa_seq in this
cpu maybe increase one, so condition "seq_retry != *seq" have always
been met.
To Fix this problem, ext4_get_discard_pa_seq_sum function couldn't add
own's cpu "discard_pa_seq" value.

Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
Signed-off-by: Ye Bin <yebin10@...wei.com>
---
 fs/ext4/mballoc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 132c118d12e1..f386fe62727d 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -372,11 +372,13 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac);
 static DEFINE_PER_CPU(u64, discard_pa_seq);
 static inline u64 ext4_get_discard_pa_seq_sum(void)
 {
-	int __cpu;
+	int __cpu, this_cpu;
 	u64 __seq = 0;
 
+	this_cpu = smp_processor_id();
 	for_each_possible_cpu(__cpu)
-		__seq += per_cpu(discard_pa_seq, __cpu);
+		if (this_cpu != __cpu)
+			__seq += per_cpu(discard_pa_seq, __cpu);
 	return __seq;
 }
 
-- 
2.25.4