From owner-freebsd-current  Wed Jan 17 10: 5:47 2001
Delivered-To: freebsd-current@freebsd.org
Received: from fw.wintelcom.net (ns1.wintelcom.net [209.1.153.20])
	by hub.freebsd.org (Postfix) with ESMTP
	id 7658C37B404; Wed, 17 Jan 2001 10:05:18 -0800 (PST)
Received: (from bright@localhost)
	by fw.wintelcom.net (8.10.0/8.10.0) id f0HI5HD24831;
	Wed, 17 Jan 2001 10:05:17 -0800 (PST)
Date: Wed, 17 Jan 2001 10:05:17 -0800
From: Alfred Perlstein <bright@wintelcom.net>
To: Randell Jesup <rjesup@wgate.com>, dillon@freebsd.org
Cc: Soren Schmidt <sos@freebsd.dk>, arch@freebsd.org,
	current@freebsd.org
Subject: Re: HEADS-UP: await/asleep removal imminent
Message-ID: <20010117100516.Q7240@fw.wintelcom.net>
References: <200101171138.MAA11834@freebsd.dk> <ybug0iixiee.fsf@jesup.eng.tvol.net.jesup.eng.tvol.net> <20010117092109.O7240@fw.wintelcom.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.2.5i
In-Reply-To: <20010117092109.O7240@fw.wintelcom.net>; from bright@wintelcom.net on Wed, Jan 17, 2001 at 09:21:09AM -0800
Sender: owner-freebsd-current@FreeBSD.ORG
Precedence: bulk
X-Loop: FreeBSD.ORG

* Alfred Perlstein <bright@wintelcom.net> [010117 09:24] wrote:
> 
> I'm not going to axe it for a few days, this is a really amazing
> API that Matt added, the problem is utility and useage over code
> complexity.
> 
> It's just a proposal.

I found several places where it may be useful, but I'm not sure if the
benefits outweigh the gains.

In a copy of the tree I've locked down the socket layer (not the entire
stack, just sockets :) ) there's code like this:

            SOCKBUF_UNLOCK(&so->so_snd, 0);
            if (top == 0) {
                MGETHDR(m, M_TRYWAIT, MT_DATA);
                if (m == NULL) {
                    error = ENOBUFS;
                    SOCKBUF_LOCK(&so->so_snd, 0);
                    goto release;
                }
                mlen = MHLEN;
...
            SOCKBUF_LOCK(&so->so_snd, 0);       /* XXX */

The lock must be unwound becasue we're calling MGETHDR with M_TRYWAIT.
If wae used M_TRY'A'WAIT the code would probably look something like
this:

            /* SOCKBUF_UNLOCK(&so->so_snd, 0); */
again:
            if (top == 0) {
                MGETHDR(m, M_TRYWAIT, MT_DATA);
                if (m == NULL) {
                    error = mawait(&so->so_snd.sb_mtx, -1, -1);
                    if (error) {
                      if (error == EWOULDBLOCK)
                         error = ENOBUFS;
                      goto release;
                    }
                    goto again;
                    /* SOCKBUF_LOCK(&so->so_snd, 0); */
                }
                mlen = MHLEN;
...
            /* SOCKBUF_LOCK(&so->so_snd, 0); */      /* XXX */

Which means we don't have to drop the lock over the socket unless
we'd block on allocation.

Matt, is this what you intended for it to do?  So far I've only 
seen it used to avoid races, but not to optimize out mutex
aquire/release.

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
"I have the heart of a child; I keep it in a jar on my desk."


To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-current" in the body of the message