On Thu, May 16, 2013 at 12:31:11PM +0800, Miao Xie wrote:
> On thu, 16 May 2013 11:36:46 +0800, Liu Bo wrote:
> > On Wed, May 15, 2013 at 03:48:20PM +0800, Miao Xie wrote:
> >> The grab/put funtions will be used in the next patch, which need grab
> >> the root object and ensure it is not freed. We use reference counter
> >> instead of the srcu lock is to aovid blocking the memory reclaim task,
> >> which invokes synchronize_srcu().
> >>
> >
> > I don't think this is necessary, we put 'kfree(root)' because we really
> > need to free them at the very end time, when there should be no inodes
> > linking on the root(we should have cleaned all inodes out from it).
> >
> > So when we flush delalloc inodes and wait for ordered extents to finish,
> > the root should be valid, otherwise someone is doing wrong things.
> >
> > And even with this grab_fs_root to avoid freeing root, it's just the
> > root that remains in memory, all its attributes, like root->node,
> > commit_root, root->inode_tree, are already NULL or empty.
>
> Please consider the case:
> Task1 Task2 Cleaner
> get the root
> flush all delalloc inodes
> drop subvolume
> iput the last inode
> move the root into the dead list
> drop
> subvolume
>
> kfree(root)
> If Task1 accesses the root now, oops will happen.
Then it's task1's fault, why it is not protected by subvol_srcu section when
it's possible that someone like task2 sets root's refs to 0?
synchronize_srcu(subvol_srcu) before adding root into dead root list is
just for this race case, why do we need another?
thanks,
liubo
>
> I introduced there two functions just to protect the access of the root
> object, not its
> attributes, so don't worry about the attributes. (Please see the first
> sentence of the
> changelog.)
>
> Thanks
> Miao
>
> >
> > thanks,
> > liubo
> >
> >> Signed-off-by: Miao Xie <mi...@cn.fujitsu.com>
> >> ---
> >> fs/btrfs/ctree.h | 1 +
> >> fs/btrfs/disk-io.c | 5 +++--
> >> fs/btrfs/disk-io.h | 21 +++++++++++++++++++++
> >> fs/btrfs/extent-tree.c | 2 +-
> >> 4 files changed, 26 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> >> index 845b77f..958ce6c 100644
> >> --- a/fs/btrfs/ctree.h
> >> +++ b/fs/btrfs/ctree.h
> >> @@ -1739,6 +1739,7 @@ struct btrfs_root {
> >> int force_cow;
> >>
> >> spinlock_t root_item_lock;
> >> + atomic_t refs;
> >> };
> >>
> >> struct btrfs_ioctl_defrag_range_args {
> >> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> >> index 42d6ba2..642c861 100644
> >> --- a/fs/btrfs/disk-io.c
> >> +++ b/fs/btrfs/disk-io.c
> >> @@ -1216,6 +1216,7 @@ static void __setup_root(u32 nodesize, u32 leafsize,
> >> u32 sectorsize,
> >> atomic_set(&root->log_writers, 0);
> >> atomic_set(&root->log_batch, 0);
> >> atomic_set(&root->orphan_inodes, 0);
> >> + atomic_set(&root->refs, 1);
> >> root->log_transid = 0;
> >> root->last_log_commit = 0;
> >> extent_io_tree_init(&root->dirty_log_pages,
> >> @@ -2049,7 +2050,7 @@ static void del_fs_roots(struct btrfs_fs_info
> >> *fs_info)
> >> } else {
> >> free_extent_buffer(gang[0]->node);
> >> free_extent_buffer(gang[0]->commit_root);
> >> - kfree(gang[0]);
> >> + btrfs_put_fs_root(gang[0]);
> >> }
> >> }
> >>
> >> @@ -3415,7 +3416,7 @@ static void free_fs_root(struct btrfs_root *root)
> >> kfree(root->free_ino_ctl);
> >> kfree(root->free_ino_pinned);
> >> kfree(root->name);
> >> - kfree(root);
> >> + btrfs_put_fs_root(root);
> >> }
> >>
> >> void btrfs_free_fs_root(struct btrfs_root *root)
> >> diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
> >> index 534d583..b71acd6e 100644
> >> --- a/fs/btrfs/disk-io.h
> >> +++ b/fs/btrfs/disk-io.h
> >> @@ -76,6 +76,27 @@ void btrfs_btree_balance_dirty_nodelay(struct
> >> btrfs_root *root);
> >> void btrfs_drop_and_free_fs_root(struct btrfs_fs_info *fs_info,
> >> struct btrfs_root *root);
> >> void btrfs_free_fs_root(struct btrfs_root *root);
> >> +
> >> +/*
> >> + * This function is used to grab the root, and avoid it is freed when we
> >> + * access it. But it doesn't ensure that the tree is not dropped.
> >> + *
> >> + * If you want to ensure the whole tree is safe, you should use
> >> + * fs_info->subvol_srcu
> >> + */
> >> +static inline struct btrfs_root *btrfs_grab_fs_root(struct btrfs_root
> >> *root)
> >> +{
> >> + if (atomic_inc_not_zero(&root->refs))
> >> + return root;
> >> + return NULL;
> >> +}
> >> +
> >> +static inline void btrfs_put_fs_root(struct btrfs_root *root)
> >> +{
> >> + if (atomic_dec_and_test(&root->refs))
> >> + kfree(root);
> >> +}
> >> +
> >> void btrfs_mark_buffer_dirty(struct extent_buffer *buf);
> >> int btrfs_buffer_uptodate(struct extent_buffer *buf, u64 parent_transid,
> >> int atomic);
> >> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> >> index 08e42c8..08f9862 100644
> >> --- a/fs/btrfs/extent-tree.c
> >> +++ b/fs/btrfs/extent-tree.c
> >> @@ -7463,7 +7463,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
> >> } else {
> >> free_extent_buffer(root->node);
> >> free_extent_buffer(root->commit_root);
> >> - kfree(root);
> >> + btrfs_put_fs_root(root);
> >> }
> >> out_end_trans:
> >> btrfs_end_transaction_throttle(trans, tree_root);
> >> --
> >> 1.8.1.4
> >>
> >> --
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>
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