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@xxxxxxxxx>
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);
