__unhash_process acquires the dcache_lock while holding the 
tasklist_lock for writing. This can deadlock. While trying to fix that 
race, I found further races with proc_dentry.
The attached patch (hopefully) fixes all these races.

Changes:
- fs/proc/base.c assumed that p->pid is reset to 0 during exit. This is 
not the case anymore. I now look at the count of the pid structure for 
PIDTYPE_PID.
- pid_delete_dentry and pid_revalidate removed. The implementation 
didn't work, and noone complained.
- only d_add the new proc entry if the task is still valid: d_add+d_drop 
is a race, if a lookup happens between both calls.
- add a new spinlock that protects proc_dentry.

What do you think? I don't like the new spinlock, but I couldn't find 
another solution.

--
    Manfred