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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 4 Jul 2022 12:16:24 -0700
Message-ID: <CAHk-=wjeEre7eeWSwCRy2+ZFH8js4u22+3JTm6n+pY-QHdhbYw@mail.gmail.com>
Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into()
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Alexander Potapenko <glider@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Mon, Jul 4, 2022 at 12:03 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> Anyway, I've thrown a mount_lock check in there, running xfstests to
> see how it goes...

So my reaction had been that it would be good to just do something like this:

  diff --git a/fs/namei.c b/fs/namei.c
  index 1f28d3f463c3..25c4bcc91142 100644
  --- a/fs/namei.c
  +++ b/fs/namei.c
  @@ -1493,11 +1493,18 @@ static bool __follow_mount_rcu(struct n...
      if (flags & DCACHE_MOUNTED) {
          struct mount *mounted = __lookup_mnt(path->mnt, dentry);
          if (mounted) {
  +           struct dentry *old_dentry = dentry;
  +           unsigned old_seq = *seqp;
  +
              path->mnt = &mounted->mnt;
              dentry = path->dentry = mounted->mnt.mnt_root;
              nd->state |= ND_JUMPED;
              *seqp = read_seqcount_begin(&dentry->d_seq);
              *inode = dentry->d_inode;
  +
  +           if (read_seqcount_retry(&old_dentry->d_seq, old_seq))
  +               return false;
  +
              /*
               * We don't need to re-check ->d_seq after this
               * ->d_inode read - there will be an RCU delay

but the above is just whitespace-damaged random monkey-scribbling by
yours truly.

More like a "shouldn't we do something like this" than a serious
patch, in other words.

IOW, it has *NOT* had a lot of real thought behind it. Purely a
"shouldn't we always clearly check the old sequence number after we've
picked up the new one?"

                   Linus