linux-kernel - Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20090216204902.GA6924@redhat.com>
Date:	Mon, 16 Feb 2009 21:49:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())

I am trying to understand the barriers in smp.c, please help!

"generic-ipi: fix the smp_mb() placement" commit
561920a0d2bb6d63343e83acfd784c0a77bd28d1 added smp_read_barrier_depends()
to generic_smp_call_function_single_interrupt().

Why it is needed? The comment says:

	/*
	 * Need to see other stores to list head for checking whether
	 * list is empty without holding q->lock
	 */
	smp_read_barrier_depends();
	while (!list_empty(&q->list)) {

But we can't miss the addition to the call_single_queue.list,
if generic_exec_single() sees list_empty(&dst->list) it sends
another IPI?

This commit also removed the barrier from csd_flag_wait(), is this OK?
Without the barrier, csd_flag_wait() can return before we see the result
of data->func() ?

IOW,
	int VAR = 0;

	void func(coid *unused)
	{
		VAR = 1;
	}

Now,

	smp_call_function_single(0, func, NULL, 1);
	BUG_ON(VAR == 0);

afaics, the BUG_ON() above is possible. Is this OK ?

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/