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Date: Fri, 24 Apr 2009 19:08:30 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       torvalds@osdl.org, Andrew Morton <akpm@linux-foundation.org>,
       serue@us.ibm.com, viro@zeniv.linux.org.uk,
       Nick Piggin <nickpiggin@yahoo.com.au>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] It may not be assumed that wake_up(), finish_wait()
	and co. imply a memory barrier
Message-ID: <20090424170830.GA13026@redhat.com>
References: <32260.1239658818@redhat.com> <20090413214852.GA1127@redhat.com> <1239659841.16771.26.camel@heimdal.trondhjem.org> <20090413222451.GA2758@redhat.com> <14561.1239873018@redhat.com> <21239.1240407420@redhat.com> <5591.1240417398@redhat.com> <21209.1240504344@redhat.com> <26028.1240573601@redhat.com> <20090424150809.GA6754@linux.vnet.ibm.com>
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On 04/24, Paul E. McKenney wrote:
>
> One question, assuming that this documentation intends to guide the
> reader on where to put the locking and/or memory-barrier primitives...
>
> Suppose we have the following sequence of events:
>
> 1.	The waiter does "set_current_state(TASK_UNINTERRUPTIBLE);".
> 	This implies a full memory barrier.
>
> 2.	The awakener updates some shared state.
>
> 3.	The awakener does "event_indicated = 1;".
>
> 4.	The waiter does "if (event_indicated)", and, finding that
> 	the event has in fact been indicated, does "break".
>
> 5.	The waiter accesses the shared state set in #2 above.
>
> 6.	Some time later, the awakener does "wake_up(&event_wait_queue);"
> 	This does not awaken anyone, so no memory barrier.
>
> 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.  Less likely (but still possible) optimizations might
> cause the waiter to access the shared state in #5 before checking
> the event_indicated flag in #4.

Do you mean something like

	awakener:

		DATA = value;
		DATA_IS_READY = true;
		wake_up(wq);


	waiter:

		set_current_state(UNINTERRUPTIBLE);
		if (DATA_IS_READY)
			do_something(DATA);

?

Imho, the code above is just buggy and should be ignored by documentation ;)

Or do I miss your point?

Oleg.