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SubjectRe: fs: lockup on rename_mutex in fs/dcache.c:1035
[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.

> Hmm... Actually, the comment in there is simply wrong - if the child
> got killed between unlocking the child and locking the parent, it's
> not ascending to the wrong parent, it's having no way to find the next
> sibling.
>
> OK, so basically it came from Nick's "fs: dcache avoid starvation in dcache
> multi-step operations" and mistake was in the assumption that once we
> hold rename_lock, nothing is going to need rename_retry. Which isn't
> true - dentry_kill() on child while we are trying to get ->d_lock on
> parent requires a restart and that isn't excluded by rename_lock at
> all.
>
> Well, brute-force fix would be this, but I wonder if it's going to
> create livelocks...
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 3ffef7f..e3d8499 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1118,6 +1118,7 @@ out_unlock:
> return;
>
> rename_retry:
> + done_seqretry(&rename_lock, seq);
> if (!retry)
> return;
> seq = 1;
> --
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