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

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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.

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