[PATCH] Fix proc_pid_lookup vs exit race

[PATCH] Fix proc_pid_lookup vs exit race

From: Manfred Spraul <manfred@colorfullife.com>

Fixes a race between proc_pid_lookup and sys_exit.

- The inodes and dentries for /proc/<pid>/whatever are cached in the dentry
  cache.  d_revalidate is used to protect against stale data: d_revalidate
  returns invalid if the task exited.

  Additionally, sys_exit flushes the dentries for the task that died -
  otherwise the dentries would stay around until they arrive at the end of
  the LRU, which could take some time.  But there is one race:

  - proc_pid_lookup finds a task and prepares new dentries for it. It must
    drop all locks for that operation.
  - the process exits, and the /proc/ dentries are flushed. Nothing
    happens, because they are not yet in the hash tables.
  - proc_pid_lookup adds the task to the dentry cache.

  Result: dentry of a dead task in the hash tables.

  The patch fixes that problem by flushing again if proc_pid_lookup notices
  that the thread exited while it created the dentry.  The patch should go
  in, but it's not critical.

- task->proc_dentry must be the dentry of /proc/<pid>.  That way sys_exit
  can flush the whole subtree at exit time.  proc_task_lookup is a direct
  copy of proc_pid_lookup and handles /proc/<>/task/<pid>.  It contains the
  lines that set task->proc_dentry.  This is bogus, and must be removed.

  This hunk is much more critical, because creates a de-facto dentry leak
  (they are recovered after flushing real dentries from the cache).