pbd_delwrite_lock and pb_list in page_buf.c

["Dale J. Stephenson" <dalestephenson@mac.com>]

I was looking at a misbehaving xfs_freeze command in an older version 
of XFS and found myself stuck in the list_for_each_safe() loop in 
pagebuf_delwri_flush.   The head had apparently been removed from the 
list in a case of spectacularly poor timing.  I found that the list had 
been protected by the pbd_delwrite_lock, but not everywhere.

Checking the current CVS, I still see that the list is still not 
protected everywhere.  Examples:

pagebuf_daemon, line 2083, calls list_del_init(&pb->pb_list) without 
holding the lock.
pagebuf_delwri_flush, line 2182, calls list_del_init(&pb->pb_list) 
without holding the lock.
pagebuf_delwri_flush, lines 2151-2166, releases the lock inside a 
list_for_each_safe loop.

I don't see any downside to putting locks around the list_del_init 
calls.  I'm not sure what to do with the last one.  If it were changed 
to list_for_each, would it survive the head getting taken away?  I'm 
assuming the lock cannot be held through __pagebuf_iorequest() or 
pagebuf_run_queues() calls.

Dale J. Stephenson
dalestephenson@mac.com