Re: [PATCH] nfsd: don't put blocked locks on LRU until after vfs_lock_file returns

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

On Tue, Nov 23, 2021 at 11:31:01AM -0500, Jeff Layton wrote:
> On Tue, 2021-11-23 at 10:59 -0500, J. Bruce Fields wrote:
> > On Tue, Nov 23, 2021 at 07:22:23AM -0500, Jeff Layton wrote:
> > > Vasily reported a case where vfs_lock_file took a very long time to
> > > return (longer than a lease period). The laundromat eventually ran and
> > > reaped the thing and when the vfs_lock_file returned, it ended up
> > > accessing freed memory.
> > 
> > By the way, once we've called vfs_lock_file(), is there anything
> > preventing nfsd4_cb_notify_lock_release() from freeing nbl before we get
> > here?
> > 
> 
> No, I don't think there is. Good catch.

It may be a rare race (an rpc's not normally going to reply in that
time), but I wouldn't be surprised if there's some error condition where
it's possible.

> Hmm...the only way I can see to fix that would be to add a refcount to
> these things, in which case we probably don't need this patch since it
> would prevent the original issue as well...

Depending on how long that might take, I'd be OK with applying this as a
stopgap.

--b.

> 
> > > 
> > > Don't put entries onto the LRU until vfs_lock_file returns.
> > > 
> > > Reported-by: Vasily Averin <vvs@virtuozzo.com>
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > >  fs/nfsd/nfs4state.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > > index bfad94c70b84..8cfef84b9355 100644
> > > --- a/fs/nfsd/nfs4state.c
> > > +++ b/fs/nfsd/nfs4state.c
> > > @@ -6966,10 +6966,8 @@ nfsd4_lock(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> > >  	}
> > >  
> > >  	if (fl_flags & FL_SLEEP) {
> > > -		nbl->nbl_time = ktime_get_boottime_seconds();
> > >  		spin_lock(&nn->blocked_locks_lock);
> > >  		list_add_tail(&nbl->nbl_list, &lock_sop->lo_blocked);
> > > -		list_add_tail(&nbl->nbl_lru, &nn->blocked_locks_lru);
> > >  		spin_unlock(&nn->blocked_locks_lock);
> > >  	}
> > >  
> > > @@ -6982,6 +6980,10 @@ nfsd4_lock(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> > >  			nn->somebody_reclaimed = true;
> > >  		break;
> > >  	case FILE_LOCK_DEFERRED:
> > > +		nbl->nbl_time = ktime_get_boottime_seconds();
> > > +		spin_lock(&nn->blocked_locks_lock);
> > > +		list_add_tail(&nbl->nbl_lru, &nn->blocked_locks_lru);
> > > +		spin_unlock(&nn->blocked_locks_lock);
> > >  		nbl = NULL;
> > >  		fallthrough;
> > >  	case -EAGAIN:		/* conflock holds conflicting lock */
> > > -- 
> > > 2.33.1
> 
> -- 
> Jeff Layton <jlayton@kernel.org>