Contention on the dentry cache lock when umounting NFS volumes on Red Hat Enterprise Linux 5

Issue

Lock contention on the dcache_lock causes I/O problems host, with soft lockup warnings similar to:

BUG: soft lockup - CPU#10 stuck for 10s! [M1IF:3779]

Call Trace:
 [<ffffffff8000c31a>] _atomic_dec_and_lock+0x39/0x57
 [<ffffffff8000d052>] dput+0x2c/0x114
 [<ffffffff80022b12>] path_release+0xc/0x24
 [<ffffffff800285ff>] vfs_stat_fd+0x3c/0x4a
 [<ffffffff8003aa41>] hrtimer_try_to_cancel+0x4a/0x53
 [<ffffffff8005a21d>] hrtimer_cancel+0xc/0x16
 [<ffffffff80063c5d>] do_nanosleep+0x47/0x70
 [<ffffffff8005a10a>] hrtimer_nanosleep+0x58/0x118
 [<ffffffff800233f9>] sys_newstat+0x19/0x31
 [<ffffffff8005d28d>] tracesys+0xd5/0xe0

The dcache_lock protects modification of the in kernel dentry cache. A dentry is an in memory data structure describing a directory entry on disk.