Re: [PATCH v4 001/100] nfsd: close potential race between delegation break and laundromat

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Jul 08, 2014 at 02:02:49PM -0400, Jeff Layton wrote:
> Bruce says:
> 
>     There's also a preexisting expire_client/laundromat vs break race:
> 
>     - expire_client/laundromat adds a delegation to its local
>       reaplist using the same dl_recall_lru field that a delegation
>       uses to track its position on the recall lru and drops the
>       state lock.
> 
>     - a concurrent break_lease adds the delegation to the lru.
> 
>     - expire/client/laundromat then walks it reaplist and sees the
>       lru head as just another delegation on the list....
> 
> Fix this race by checking the dl_time under the state_lock. If we find
> that it's not 0, then we know that it has already been queued to the LRU
> list and that we shouldn't queue it again.
> 
> In the case of destroy_client, we must also ensure that we don't hit
> similar races by ensuring that we don't move any delegations to the
> reaplist with a dl_time of 0. Just bump the dl_time by one before we
> drop the state_lock. We're destroying the delegations anyway, so a 1s
> difference there won't matter.
> 
> The fault injection code also requires a bit of surgery here:
> 
> First, in the case of nfsd_forget_client_delegations, we must prevent
> the same sort of race vs. the delegation break callback. For that, we
> just increment the dl_time to ensure that a delegation callback can't
> race in while we're working on it.
> 
> We can't do that for nfsd_recall_client_delegations, as we need to have
> it actually queue the delegation, and that won't happen if we increment
> the dl_time. The state lock is held over that function, so we don't need
> to worry about these sorts of races there.
> 
> There is one other potential bug nfsd_recall_client_delegations though.
> Entries on the victims list are not dequeued before calling
> nfsd_break_one_deleg. That's a potential list corruptor, so ensure that
> we do that there.
> 
> Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
> Signed-off-by: Jeff Layton <jlayton@primarydata.com>
> ---
>  fs/nfsd/nfs4state.c | 40 +++++++++++++++++++++++++++++++++-------
>  1 file changed, 33 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 763aeeb67ccf..633b34fd6c92 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1287,6 +1287,8 @@ destroy_client(struct nfs4_client *clp)
>  	while (!list_empty(&clp->cl_delegations)) {
>  		dp = list_entry(clp->cl_delegations.next, struct nfs4_delegation, dl_perclnt);
>  		list_del_init(&dp->dl_perclnt);
> +		/* Ensure that deleg break won't try to requeue it */
> +		++dp->dl_time;
>  		list_move(&dp->dl_recall_lru, &reaplist);
>  	}
>  	spin_unlock(&state_lock);
> @@ -2933,10 +2935,14 @@ static void nfsd_break_one_deleg(struct nfs4_delegation *dp)
>  	 * it's safe to take a reference: */
>  	atomic_inc(&dp->dl_count);
>  
> -	list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
> -
> -	/* Only place dl_time is set; protected by i_lock: */
> -	dp->dl_time = get_seconds();
> +	/*
> +	 * If the dl_time != 0, then we know that it has already been
> +	 * queued for a lease break. Don't queue it again.
> +	 */
> +	if (dp->dl_time == 0) {

Any reason not to just make that

	if (dp->dl_time)
		return;

?

I don't know if it matters right now, but it might also be useful to
know that no more work will get queued once dl_time is set.

--b.

> +		list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
> +		dp->dl_time = get_seconds();
> +	}
>  
>  	block_delegations(&dp->dl_fh);
>  
> @@ -5081,8 +5087,23 @@ static u64 nfsd_find_all_delegations(struct nfs4_client *clp, u64 max,
>  
>  	lockdep_assert_held(&state_lock);
>  	list_for_each_entry_safe(dp, next, &clp->cl_delegations, dl_perclnt) {
> -		if (victims)
> +		if (victims) {
> +			/*
> +			 * It's not safe to mess with delegations that have a
> +			 * non-zero dl_time. They might have already been broken
> +			 * and could be processed by the laundromat outside of
> +			 * the state_lock. Just leave them be.
> +			 */
> +			if (dp->dl_time != 0)
> +				continue;
> +
> +			/*
> +			 * Increment dl_time to ensure that delegation breaks
> +			 * don't monkey with it now that we are.
> +			 */
> +			++dp->dl_time;
>  			list_move(&dp->dl_recall_lru, victims);
> +		}
>  		if (++count == max)
>  			break;
>  	}
> @@ -5107,14 +5128,19 @@ u64 nfsd_forget_client_delegations(struct nfs4_client *clp, u64 max)
>  
>  u64 nfsd_recall_client_delegations(struct nfs4_client *clp, u64 max)
>  {
> -	struct nfs4_delegation *dp, *next;
> +	struct nfs4_delegation *dp;
>  	LIST_HEAD(victims);
>  	u64 count;
>  
>  	spin_lock(&state_lock);
>  	count = nfsd_find_all_delegations(clp, max, &victims);
> -	list_for_each_entry_safe(dp, next, &victims, dl_recall_lru)
> +	while (!list_empty(&victims)) {
> +		dp = list_first_entry(&victims, struct nfs4_delegation,
> +					dl_recall_lru);
> +		list_del_init(&dp->dl_recall_lru);
> +		dp->dl_time = 0;
>  		nfsd_break_one_deleg(dp);
> +	}
>  	spin_unlock(&state_lock);
>  
>  	return count;
> -- 
> 1.9.3
>