| Message ID | 20201128174124.22397-1-shacharr@gmail.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | f2fs: Fix deadlock between f2fs_quota_sync and block_operation | expand |
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); > >
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);
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. 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(-)