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Btrfs: race free update of commit root for ro snapshots

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This is a better solution for the problem addressed in the following
commit:

    Btrfs: update commit root on snapshot creation after orphan cleanup
    (3821f34888)

The previous solution wasn't the best because of 2 reasons:

    1) It added another full transaction commit, which is more expensive
       than just swapping the commit root with the root;

    2) If a reboot happened after the first transaction commit (the one
       that creates the snapshot) and before the second transaction commit,
       then we would end up with the same problem if a send using that
       snapshot was requested before the first transaction commit after
       the reboot.

This change addresses those 2 issues. The second issue is addressed by
switching the commit root in the dentry lookup VFS callback, which is
also called by the snapshot/subvol creation ioctl and performs orphan
cleanup if needed. Like the vfs, the ioctl locks the parent inode too,
preventing race issues between a dentry lookup and snapshot creation.

Cc: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
This commit is contained in:
Filipe Manana 2014-07-31 14:41:07 +01:00 • committed by Chris Mason
parent 87fa3bb078
commit 9c3b306e1c
2 changed files with 36 additions and 33 deletions
Download patch file Download diff file Expand all files Collapse all files

33
fs/btrfs/ioctl.c
View file

@ -711,39 +711,6 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
if (ret)
goto fail;
ret = btrfs_orphan_cleanup(pending_snapshot->snap);
if (ret)
goto fail;
/*
* If orphan cleanup did remove any orphans, it means the tree was
* modified and therefore the commit root is not the same as the
* current root anymore. This is a problem, because send uses the
* commit root and therefore can see inode items that don't exist
* in the current root anymore, and for example make calls to
* btrfs_iget, which will do tree lookups based on the current root
* and not on the commit root. Those lookups will fail, returning a
* -ESTALE error, and making send fail with that error. So make sure
* a send does not see any orphans we have just removed, and that it
* will see the same inodes regardless of whether a transaction
* commit happened before it started (meaning that the commit root
* will be the same as the current root) or not.
*/
if (readonly && pending_snapshot->snap->node !=
pending_snapshot->snap->commit_root) {
trans = btrfs_join_transaction(pending_snapshot->snap);
if (IS_ERR(trans) && PTR_ERR(trans) != -ENOENT) {
ret = PTR_ERR(trans);
goto fail;
}
if (!IS_ERR(trans)) {
ret = btrfs_commit_transaction(trans,
pending_snapshot->snap);
if (ret)
goto fail;
}
}
inode = btrfs_lookup_dentry(dentry->d_parent->d_inode, dentry);
if (IS_ERR(inode)) {
ret = PTR_ERR(inode);