linux-kernel - Re: ipc,sem: sysv semaphore scalability

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Date:	Fri, 29 Mar 2013 12:26:45 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	hhuang@...hat.com, "Low, Jason" <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Larry Woodman <lwoodman@...hat.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	Peter Hurley <peter@...leysoftware.com>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>
Subject: Re: ipc,sem: sysv semaphore scalability

On Fri, Mar 29, 2013 at 12:06 PM, Dave Jones <davej@...hat.com> wrote:
>
> Here's an oops I just hit..
>
> BUG: unable to handle kernel NULL pointer dereference at 000000000000000f
> IP: [<ffffffff812c24ca>] testmsg.isra.5+0x1a/0x60

Btw, looking at the code leading up to this, what the f*ck is wrong
with the IPC stuff?

It's using the generic list stuff for most of the lists, but then it
open-codes the accesses.

So instead of using

   for_each_entry(walk_msg, &msq->q_messages, m_list) {
      ..
   }

the ipc/msg.c code does all that by hand, with

   tmp = msq->q_messages.next;
   while (tmp != &msq->q_messages) {
      struct msg_msg *walk_msg;

      walk_msg = list_entry(tmp, struct msg_msg, m_list);
      ...
      tmp = tmp->next;
   }

Ugh. The code is near unreadable. And then it has magic memory
barriers etc, implying that it doesn't lock the data structures, but
no comments about them. See expunge_all() and pipelined_send().

The code seems entirely random, and it's badly set up (annoyance of
the day: crazy helper functions in ipc/msgutil.c to make sure that (a)
you have to spend more effort looking for them, and (b) they won't get
inlined).

Clearly nobody has cared for the crazy IPC message code in a long time.

              Linus
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