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

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Date:	Fri, 24 Apr 2009 19:08:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	David Howells <dhowells@...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

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.

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