Re: fs: lockup on rename_mutex in fs/dcache.c:1035

[Linus Torvalds]

On Sat, Oct 25, 2014 at 8:57 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> [context for Linus]
>
> Fuzzer has triggered deadlock in d_walk() with rename_lock taken twice.
> AFAICS, the plausible scenario is
>                          (child->d_flags & DCACHE_DENTRY_KILLED) ||
> triggering while ascending to parent, on the pass with rename_lock already
> held exclusive.  In that case we go to rename_retry and either return without
> unlocking rename_lock, or try to take in exclusive one more time, again
> without unlocking it first.

Your patch looks fine, and I don't think we can livelock - because we
always set 'seq' to 1 if we retry, and that causes us to get the
exclusive lock, so we'd better not then enter the retry loop again.
Although I guess not only renames cause it - looks like we get to that
mis-named "rename_retry" for a deletion too according to the comment.
Although I'm not sure that comment is correct - I don't think we
change d_parent for deletes, do we?

So at a quick glance, the one-liner patch looks fine. That said, I
really detest how that code is written. Especially if the rename_retry
really can only happen once, I get the feeling that we would be *much*
better off doing this explicitly with a wrapper function, and do
something like

   void d_walk(,..)
   {
       /* Try non-exclusive first */
       seq = read_seqbegin(lock);
       retry = __d_walk(.. seq);
       if (retry) {
              read_seqlock_excl(lock);
              // lock->seqcount is now guaranteed stable
              retry = __d_walk(.. lock->seqcount);
              read_sequnlock_excl(lock);
              WARN_ON_ONCE(retry, "Really?");
       }
   }

or whatever. But maybe I'm missing some reason why the above is just
crazy, and I'm a moron. Entirely possible.

                     Linus
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