Re: [Bug #11989] Suspend failure on NForce4-based boards due to chanes in stop_machine

[Vegard Nossum]

On Tue, Nov 11, 2008 at 2:36 PM, Vegard Nossum <vegard.nossum@xxxxxxxxx> wrote:
> On Tue, Nov 11, 2008 at 11:52 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
>> [ Cc:-ed workqueue/locking/suspend-race-condition experts. ]
>
> Heh. I am not expert, but I looked at the code. The obvious suspicious
> thing to see is the use of unpaired barriers? Maybe like this:

...

> 55 /* Last one to ack a state moves to the next state. */
> 56 static void ack_state(void)
> 57 {
> 58         if (atomic_dec_and_test(&thread_ack))
>
> Maybe
> + /* force ordering between thread_ack/state */
> + smp_rmb();
> here?

Oops, I am wrong (after a small investigation).

"1490 Any atomic operation that modifies some state in memory and
returns information
1491 about the state (old or new) implies an SMP-conditional general
memory barrier
1492 (smp_mb()) on each side of the actual operation (with the exception of
1493 explicit lock operations, described later).  These include:
1494
...
1503         atomic_dec_and_test();"

Won't fix the problem at hand, but maybe something like this would be
nice for future generations :-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 0e688c6..6796bb1 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -55,6 +55,7 @@ static void set_state(enum stopmachine_state newstate)
 /* Last one to ack a state moves to the next state. */
 static void ack_state(void)
 {
+       /* Implicit memory barrier; no smp_rmb() needed */
        if (atomic_dec_and_test(&thread_ack))
                set_state(state + 1);
 }


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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