Re: IGMP and rwlock: Dead ocurred again on TILEPro

From: Américo Wang <redacted>
Date: Thu, 17 Feb 2011 13:42:37 +0800

On Thu, Feb 17, 2011 at 01:04:14PM +0800, Cypher Wu wrote:

quoted

quoted

Have you turned CONFIG_LOCKDEP on?

I think Eric already converted that rwlock into RCU lock, thus
this problem should disappear. Could you try a new kernel?

Thanks.

I haven't turned CONFIG_LOCKDEP on for test since I didn't get too
much information when we tried to figured out the former deadlock.

IGMP used read_lock() instead of read_lock_bh() since usually
read_lock() can be called recursively, and today I've read the
implementation of MIPS, it's should also works fine in that situation.
The implementation of TILEPro cause problem since after it use TNS set
the lock-val to 1 and hold the original value and before it re-set
lock-val a new value, it a race condition window.

I see no reason why you can't call read_lock_bh() recursively,
read_lock_bh() is roughly equalent to local_bh_disable() + read_lock(),
both can be recursive.

But I may miss something here. :-/

IGMP is doing this so that taking the read lock does not stop packet
processing.

TILEPro's rwlock implementation is simply buggy and needs to be fixed.