tree d9f21bd649683bedbdadb301b1533c09e13591ba
parent da1aab3dca9aa88ae34ca392470b8943159e25fe
author NeilBrown <neilb@suse.de> 1397018031 +1000
committer NeilBrown <neilb@suse.de> 1397018554 +1000

md: avoid oops on unload if some process is in poll or select.

If md-mod is unloaded while some process is in poll() or select(),
then that process maintains a pointer to md_event_waiters, and when
the try to unlink from that list, they will oops.

The procfs infrastructure ensures that ->poll won't be called after
remove_proc_entry, but doesn't provide a wait_queue_head for us to
use, and the waitqueue code doesn't provide a way to remove all
listeners from a waitqueue.

So we need to:
 1/ make sure no further references to md_event_waiters are taken (by
    setting md_unloading)
 2/ wake up all processes currently waiting, and
 3/ wait until all those processes have disconnected from our
    wait_queue_head.

Reported-by: "majianpeng" <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>