Re: runningbufspace related lock-ups with md(4)/UFS/SU (PATCH ?)

I have been able to reproduce your hang on my system and your suggested
fix does prevent it. I am going to run some more buffer starvation-type
tests on it this week and if they do not cause other problems, I will
put in your suggested fix.

	Kirk McKusick

=-=-=-=-=-=

To: current_at_freebsd.org
From: Brian Fundakowski Feldman <green_at_freebsd.org>
Mime-Version: 1.0
Date: Thu, 16 Oct 2003 15:32:58 -0400
Cc: phk_at_freebsd.org
Subject: runningbufspace related lock-ups with md(4)/UFS/SU (PATCH ?)

I'm having problems where the entire system is locking up when using a MD 
UFS+SoftUpdates partition.  I can simply dd if=/dev/zero of=/mnt/foo and in 
a couple tries it will lock up.  When it locks up, buf_daemon (or if that is 
patched against, syncer) is calling waitrunningbufspace() from a non-B_ASYNC 
buf call.  Because of this, the md(4) ("md0") thread is stuck in "ufs" 
waiting to receive a lock on the vnode that one of the syncer/flusher 
daemons has locked, waiting for bufspace to run down.  The user program 
causing the problem is still stuck in "wdrain" because it's also waiting for 
waitrunningbufspace() to return.  In short, everything wants to try to 
reduce the amount of outstanding buffer space, but nothing moves forward 
while GEOM/md(4)/what have you are waiting for the daemons to let go of the 
vnode so they can write out data.
Does this scenario make sense?  I have fixed it here using the following 
very simple patch, which disables the implicit waitrunningbufspace() calls
so the daemons can't get stuck there.

diff -r1.412 vfs_bio.c
73a74,75
> static struct proc *bufdaemonproc;
>
889c891,893
<		waitrunningbufspace();
---
>		if (curthread->td_proc != bufdaemonproc &&
>		    curthread->td_proc != updateproc)
>			waitrunningbufspace();
2038,2039d2041
<
< static struct proc *bufdaemonproc;

-- 
Brian Fundakowski Feldman                           \'[ FreeBSD ]''''''''''\
  <> green_at_FreeBSD.org                               \  The Power to Serve! \
 Opinions expressed are my own.                       \,,,,,,,,,,,,,,,,,,,,,,\