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    Subject[PATCH] vfs: fix race in rcu lookup of pruned dentry
    Since the RCU dcache lookup came in, on five occasions a "cp -a" in my dual
    kbuild swapping tests has failed because a source file vanished momentarily
    e.g. cp: cannot open `/usr/src/linux/include/linux/cramfs_fs.h' for reading:
    No such file or directory

    That -ENOENT in walk_component: isn't it assuming we found a negative
    dentry, before reaching the read_seqcount_retry which complete_walk
    (or nameidata_drop_rcu_last before 3.0) would use to confirm a successful
    lookup? And can't memory pressure prune a dentry, coming to dentry_kill
    which __d_drops to unhash before dentry_iput resets d_inode to NULL, but
    the dentry_rcuwalk_barrier between those is ineffective if the other end
    ignores the seqcount?

    Move !inode -ENOENT checks from walk_component down into do_lookup,
    adding read_seqcount_retry confirmation in the RCU case. And use
    path_put_conditional instead of path_to_nameidata (a reverse way of
    doing the same) in the non-RCU case, to match the error case just above.

    Signed-off-by: Hugh Dickins <>
    I think I now have a faster script (four slightly staggered "cp -a"s of
    kernel source, run in limited memory) to reproduce this in hours rather
    than weeks; but I'm still calibrating that, cannot yet say whether this
    patch fixes it. Sending patch on ahead in case it's obvious to you.

    fs/namei.c | 16 +++++++++-------
    1 file changed, 9 insertions(+), 7 deletions(-)

    --- 3.0-git/fs/namei.c 2011-07-15 19:40:03.269867213 -0700
    +++ linux/fs/namei.c 2011-07-17 10:08:17.586475293 -0700
    @@ -1173,7 +1173,10 @@ static int do_lookup(struct nameidata *n
    goto unlazy;
    if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT))
    goto unlazy;
    - return 0;
    + if (*inode)
    + return 0;
    + if (!read_seqcount_retry(&dentry->d_seq, nd->seq))
    + return -ENOENT;
    if (unlazy_walk(nd, dentry))
    return -ECHILD;
    @@ -1223,6 +1226,10 @@ retry:
    return err;
    *inode = path->dentry->d_inode;
    + if (!*inode) {
    + path_put_conditional(path, nd);
    + return -ENOENT;
    + }
    return 0;

    @@ -1276,15 +1283,10 @@ static inline int walk_component(struct
    if (unlikely(type != LAST_NORM))
    return handle_dots(nd, type);
    err = do_lookup(nd, name, path, &inode);
    - if (unlikely(err)) {
    + if (err) {
    return err;
    - if (!inode) {
    - path_to_nameidata(path, nd);
    - terminate_walk(nd);
    - return -ENOENT;
    - }
    if (unlikely(inode->i_op->follow_link) && follow) {
    if (nd->flags & LOOKUP_RCU) {
    if (unlikely(unlazy_walk(nd, path->dentry))) {

     \ /
      Last update: 2011-07-17 23:07    [W:0.024 / U:75.212 seconds]
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