NFS4: PUTFH returns NFS4ERR_DELAY, nfsd threads appear stuck in cache_check/cache_wait_req

Issue

• df -k hangs on client
• in tcpdump we can observe server replies NFS4ERR_DELAY to PUTFH operation
• server threads shows the following stack:

PID: 8093   TASK: ffff8808a47c0fb0  CPU: 0   COMMAND: "nfsd"
 #0 [ffff8818453e3890] __schedule at ffffffff8168ba15
 #1 [ffff8818453e38f8] schedule at ffffffff8168c069
 #2 [ffff8818453e3908] schedule_timeout at ffffffff81689a04
 #3 [ffff8818453e39b8] wait_for_completion_interruptible_timeout at ffffffff8168c8c2
 #4 [ffff8818453e3a20] cache_wait_req at ffffffffa0543e5c [sunrpc]
 #5 [ffff8818453e3ac0] cache_check at ffffffffa0544d8c [sunrpc]
 #6 [ffff8818453e3b10] exp_get_by_name at ffffffffa06e1ddd [nfsd]
 #7 [ffff8818453e3c10] exp_find at ffffffffa06e2c50 [nfsd]
 #8 [ffff8818453e3cb8] rqst_exp_find at ffffffffa06e3ee9 [nfsd]
 #9 [ffff8818453e3cf8] fh_verify at ffffffffa06dd469 [nfsd]
#10 [ffff8818453e3d68] nfsd4_putfh at ffffffffa06ebe29 [nfsd]
#11 [ffff8818453e3d90] nfsd4_proc_compound at ffffffffa06ee70c [nfsd]
#12 [ffff8818453e3de0] nfsd_dispatch at ffffffffa06d96fb [nfsd]
#13 [ffff8818453e3e18] svc_process_common at ffffffffa0539ea3 [sunrpc]
#14 [ffff8818453e3e78] svc_process at ffffffffa053a24b [sunrpc]
#15 [ffff8818453e3ea0] nfsd at ffffffffa06d907f [nfsd]
#16 [ffff8818453e3ec8] kthread at ffffffff810b0a4f
#17 [ffff8818453e3f50] ret_from_fork at ffffffff81696fd8