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Subject[PATCH 1/2] f2fs: avoid race in between atomic_read & atomic_inc
Date
Previously, we will miss merging flush command during fsync due to below
race condition:

Thread A Thread B Thread C
- f2fs_issue_flush
- atomic_read(&issing_flush)
- f2fs_issue_flush
- atomic_read(&issing_flush)
- f2fs_issue_flush
- atomic_read(&issing_flush)
- atomic_inc(&issing_flush)
- atomic_inc(&issing_flush)
- atomic_inc(&issing_flush)
- submit_flush_wait
- submit_flush_wait
- submit_flush_wait

It needs to use atomic_inc_return instead to avoid such race.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
---
fs/f2fs/segment.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index d6c3f456ea51..1215ca1bd4e2 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -536,8 +536,7 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi)
return ret;
}

- if (!atomic_read(&fcc->issing_flush)) {
- atomic_inc(&fcc->issing_flush);
+ if (atomic_inc_return(&fcc->issing_flush) == 1) {
ret = submit_flush_wait(sbi);
atomic_dec(&fcc->issing_flush);

@@ -547,7 +546,6 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi)

init_completion(&cmd.wait);

- atomic_inc(&fcc->issing_flush);
llist_add(&cmd.llnode, &fcc->issue_list);

/* update issue_list before we wake up issue_flush thread */
--
2.13.1.388.g69e6b9b4f4a9
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