linux-kernel - [PATCH 5.15 160/179] f2fs: quota: fix potential deadlock

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Message-Id: <20211129181724.206766837@linuxfoundation.org>
Date:   Mon, 29 Nov 2021 19:19:14 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Yi Zhuang <zhuangyi1@...wei.com>,
        Chao Yu <chao@...nel.org>, Jaegeuk Kim <jaegeuk@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 160/179] f2fs: quota: fix potential deadlock

From: Chao Yu <chao@...nel.org>

[ Upstream commit a5c0042200b28fff3bde6fa128ddeaef97990f8d ]

As Yi Zhuang reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=214299

There is potential deadlock during quota data flush as below:

Thread A:			Thread B:
f2fs_dquot_acquire
down_read(&sbi->quota_sem)
				f2fs_write_checkpoint
				block_operations
				f2fs_look_all
				down_write(&sbi->cp_rwsem)
f2fs_quota_write
f2fs_write_begin
__do_map_lock
f2fs_lock_op
down_read(&sbi->cp_rwsem)
				__need_flush_qutoa
				down_write(&sbi->quota_sem)

This patch changes block_operations() to use trylock, if it fails,
it means there is potential quota data updater, in this condition,
let's flush quota data first and then trylock again to check dirty
status of quota data.

The side effect is: in heavy race condition (e.g. multi quota data
upaters vs quota data flusher), it may decrease the probability of
synchronizing quota data successfully in checkpoint() due to limited
retry time of quota flush.

Reported-by: Yi Zhuang <zhuangyi1@...wei.com>
Signed-off-by: Chao Yu <chao@...nel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/f2fs/checkpoint.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index 83e9bc0f91ffd..7b02827242312 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -1162,7 +1162,8 @@ static bool __need_flush_quota(struct f2fs_sb_info *sbi)
 	if (!is_journalled_quota(sbi))
 		return false;
 
-	down_write(&sbi->quota_sem);
+	if (!down_write_trylock(&sbi->quota_sem))
+		return true;
 	if (is_sbi_flag_set(sbi, SBI_QUOTA_SKIP_FLUSH)) {
 		ret = false;
 	} else if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR)) {
-- 
2.33.0