proc_lookup and task exiting are not synchronized, although some of the
previous code may have suggested that.  Every time before we reuse a dentry
namei.c calls d_op->derevalidate which prevents us from reusing a stale dcache
entry.  Unfortunately it does not prevent us from returning a stale dcache
entry.  This race has been explicitly plugged in proc_pid_lookup but there is
nothing to confine it to just that proc lookup function.

So to prevent the race I call revalidate explictily in all of the proc lookup
functions after I call d_add, and report an error if the revalidate does not
succeed.

Years ago Al Viro did something similar but those changes got lost in the
churn.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>