On 2020/11/29 1:41, Shachar Raindel wrote:

This deadlock is hitting Android users (Pixel 3/3a/4) with Magisk, due to frequent umount/mount operations that trigger quota_sync, hitting the race. See https://github.com/topjohnwu/Magisk/issues/3171 for additional impact discussion.

In commit db6ec53b7e03, we added a semaphore to protect quota flags. As part of this commit, we changed f2fs_quota_sync to call f2fs_lock_op, in an attempt to prevent an AB/BA type deadlock with quota_sem locking in block_operation. However, rwsem in Linux is not recursive. Therefore, the following deadlock can occur:

f2fs_quota_sync down_read(cp_rwsem) // f2fs_lock_op filemap_fdatawrite f2fs_write_data_pages ... block_opertaion down_write(cp_rwsem) - marks rwsem as "writer pending" down_read_trylock(cp_rwsem) - fails as there is a writer pending. Code keeps on trying, live-locking the filesystem.

f2fs_write_single_data_page() will not grab read lock of cp_rwsem now, could you please check f2fs code in mainline?

/* Dentry/quota blocks are controlled by checkpoint */ if (S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) { /* * We need to wait for node_write to avoid block allocation during * checkpoint. This can only happen to quota writes which can cause * the below discard race condition. */ if (IS_NOQUOTA(inode)) down_read(&sbi->node_write);

fio.need_lock = LOCK_DONE; err = f2fs_do_write_data_page(&fio);

if (IS_NOQUOTA(inode)) up_read(&sbi->node_write);

goto done; }

Thanks,

We solve this by creating a new rwsem, used specifically to synchronize this case, instead of attempting to reuse an existing lock.

Signed-off-by: Shachar Raindel shacharr@gmail.com

Fixes: db6ec53b7e03 f2fs: add a rw_sem to cover quota flag changes

fs/f2fs/f2fs.h | 3 +++ fs/f2fs/super.c | 13 +++++++++++-- 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h index cb700d797296..b3e55137be7f 100644 --- a/fs/f2fs/f2fs.h +++ b/fs/f2fs/f2fs.h @@ -1448,6 +1448,7 @@ struct f2fs_sb_info { struct inode *meta_inode; /* cache meta blocks */ struct mutex cp_mutex; /* checkpoint procedure lock */ struct rw_semaphore cp_rwsem; /* blocking FS operations */

• struct rw_semaphore cp_quota_rwsem; /* blocking quota sync operations */ struct rw_semaphore node_write; /* locking node writes */ struct rw_semaphore node_change; /* locking node change */ wait_queue_head_t cp_wait;

@@ -1961,12 +1962,14 @@ static inline void f2fs_unlock_op(struct f2fs_sb_info *sbi) static inline void f2fs_lock_all(struct f2fs_sb_info *sbi) {

• down_write(&sbi->cp_quota_rwsem); down_write(&sbi->cp_rwsem); }

static inline void f2fs_unlock_all(struct f2fs_sb_info *sbi) { up_write(&sbi->cp_rwsem);

• up_write(&sbi->cp_quota_rwsem); }

static inline int __get_cp_reason(struct f2fs_sb_info *sbi) diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c index 00eff2f51807..5ce61147d7e5 100644 --- a/fs/f2fs/super.c +++ b/fs/f2fs/super.c @@ -2209,8 +2209,16 @@ int f2fs_quota_sync(struct super_block *sb, int type) * f2fs_dquot_commit * block_operation * down_read(quota_sem)

• *
  

• * However, we cannot use the cp_rwsem to prevent this
  

• * deadlock, as the cp_rwsem is taken for read inside the
  

• * f2fs_dquot_commit code, and rwsem is not recursive.
  

• *
  

• * We therefore use a special lock to synchronize
  

• * f2fs_quota_sync with block_operations, as this is the only
  

• * place where such recursion occurs.
  

  */

• f2fs_lock_op(sbi);

• down_read(&sbi->cp_quota_rwsem);

down_read(&sbi->quota_sem); ret = dquot_writeback_dquots(sb, type); @@ -2251,7 +2259,7 @@ int f2fs_quota_sync(struct super_block *sb, int type) if (ret) set_sbi_flag(F2FS_SB(sb), SBI_QUOTA_NEED_REPAIR); up_read(&sbi->quota_sem);

• f2fs_unlock_op(sbi);

• up_read(&sbi->cp_quota_rwsem); return ret; }

@@ -3599,6 +3607,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent) init_rwsem(&sbi->cp_rwsem); init_rwsem(&sbi->quota_sem);

• init_rwsem(&sbi->cp_quota_rwsem); init_waitqueue_head(&sbi->cp_wait); init_sb_info(sbi);