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    SubjectRe: Linux 2.6.38-rc5
    On Wed, Feb 16, 2011 at 04:06:43PM +0000, Al Viro wrote:

    > Sigh... I see what's going on. We'd got inode from dentry that is getting
    > crapped under us. We will *not* survive dropping RCU - it's bad enough for
    > full restart in normal mode. So right after we'd seen that (already wrong)
    > inode has ->follow_link(), we decide to drop RCU. Originally this BUG_ON
    > hadn't been reached in that case - we had already failed with -ECHILD before
    > we got to it. Now we don't...
    >
    > _However_, I don't like passing inode to do_follow_link(). I'd rather set
    > nd->inode to inode first and use it there. Let me think a bit and see if
    > it's feasible...

    No, that won't do. The damn thing uses previous value of nd->inode if
    it walks into relative symlink...

    Let's shift that call of nameidata_dentry_drop_rcu_maybe() into both
    callers of do_follow_link() instead. Marginally less obvious that we won't
    reach the guts of do_follow_link() in RCU mode, just as obvious that overall
    structure is ugly as hell and avoids making it even uglier by passing inode
    down there. How about this:

    diff --git a/fs/namei.c b/fs/namei.c
    index 9e701e2..d7003cf 100644
    --- a/fs/namei.c
    +++ b/fs/namei.c
    @@ -800,10 +800,6 @@ static inline int do_follow_link(struct path *path, struct nameidata *nd)
    void *cookie;
    int err = -ELOOP;

    - /* We drop rcu-walk here */
    - if (nameidata_dentry_drop_rcu_maybe(nd, path->dentry))
    - return -ECHILD;
    -
    if (current->link_count >= MAX_NESTED_LINKS)
    goto loop;
    if (current->total_link_count >= 40)
    @@ -1413,6 +1409,9 @@ exec_again:
    goto out_dput;

    if (inode->i_op->follow_link) {
    + /* We drop rcu-walk here */
    + if (nameidata_dentry_drop_rcu_maybe(nd, next.dentry))
    + return -ECHILD;
    BUG_ON(inode != next.dentry->d_inode);
    err = do_follow_link(&next, nd);
    if (err)
    @@ -1458,6 +1457,8 @@ last_component:
    break;
    if (inode && unlikely(inode->i_op->follow_link) &&
    (lookup_flags & LOOKUP_FOLLOW)) {
    + if (nameidata_dentry_drop_rcu_maybe(nd, next.dentry))
    + return -ECHILD;
    BUG_ON(inode != next.dentry->d_inode);
    err = do_follow_link(&next, nd);
    if (err)

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