linux-kernel - Re: [PATCH] It may not be assumed that wake_up(), finish

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Message-ID: <27891.1240595286@redhat.com>
Date:	Fri, 24 Apr 2009 18:48:06 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, torvalds@...l.org,
	Andrew Morton <akpm@...ux-foundation.org>, serue@...ibm.com,
	viro@...iv.linux.org.uk, Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] It may not be assumed that wake_up(), finish_wait() and co. imply a memory barrier

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> Because there is no memory barrier between #2 and #3, reordering by
> either the compiler or the CPU might cause the awakener to update the
> event_indicated flag in #3 -before- completing its update of shared
> state in #2.

If the ordering of #2 and #3 is important with respect to each other, then the
awakener must manually interpolate a barrier of some sort between the two
_before_ calling wake_up() (or it should wrap them in a lock).

As I've tried to make clear in my documentation:

	Sleeping and waking on an event flagged in global data can be viewed as
	an interaction between two pieces of data: ===> the task state of the
	task waiting for the event and the global data used to indicate the
	event <===.

the barrier in wake_up() is only concerned with the ordering of #3 vs #6.  That
is all it _can_ impose an order upon, since #2 and #3 both happen before
wake_up() is called, and #3 is what causes the sleeper to break out of the
sleep loop.

> So, for this to work correctly, don't we need at least an smp_wmb()
> between #2 and #3 and at least an smp_rmb() between #4 and #5?  And if
> #2 does reads (but not writes) at least one variable in the shared state
> that #5 writes to, don't both barriers need to be smp_mb()?

Yes, but that's beyond the scope of this section.  set_current_state() imposes
the partial ordering { #1, #4 } and wake_up() the partial ordering { #3, #6 }
because those are the controlling features of the loop.

Managing the data beyond that is up to the caller of set_current_state() and
the caller of wake_up().

David
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