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

[Al Viro <viro@ZenIV.linux.org.uk>]

[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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