f2fs: quota: fix potential deadlock

BugLink: https://bugs.launchpad.net/bugs/1959437
commit a5c0042200b28fff3bde6fa128ddeaef97990f8d upstream.

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@huawei.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index f7d27cbbeb860ed0090921fd7e1c736d90f9695e..03dce3980d90a41ad4602774a88b4a98d4fcbae5 100644 (file)
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -1144,7 +1144,8 @@ 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)) {