linux-kernel - Re: [PATCH 2/2] futex: correct futex_requeue futex key ref counting in requeue loop

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Date:	Fri, 2 Jan 2009 23:13:10 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rusty Russell <rusty@....ibm.com>
Subject: Re: [PATCH 2/2] futex: correct futex_requeue futex key ref
	counting in requeue loop


* Darren Hart <dvhltc@...ibm.com> wrote:

> Darren Hart wrote:
>> The requeue loop takes multiple references to key2, but the corresponding
>> put loop decrements the refs for key1.  This patch corrects the accounting.
>>
>> Build and boot tested on an x86_64 system.
>>
>> Signed-off-by: Darren Hart <dvhltc@...ibm.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Rusty Russell <rusty@....ibm.com>
>> ---
>>
>>  kernel/futex.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/kernel/futex.c b/kernel/futex.c
>> index cf363ce..3b66d91 100644
>> --- a/kernel/futex.c
>> +++ b/kernel/futex.c
>> @@ -919,7 +919,7 @@ out_unlock:
>>
>>  	/* drop_futex_key_refs() must be called outside the spinlocks. */
>>  	while (--drop_count >= 0)
>> -		drop_futex_key_refs(&key1);
>> +		drop_futex_key_refs(&key2);
>>
>>  	put_futex_key(fshared, &key2);
>>  out_put_key1:
>>
>
> Ugh, so I'm having second thoughts about this patch.  I believe what is  
> happening here is that the requeue loop requeues each waiter from one  
> futex (key1) to another (key2).  It rightly takes a reference to the  
> futex at key2 and then decrements the references to key1 by drop_count  
> (since the waiters now reference key2, not key1).  The newly taken key2  
> references will be dropped in futex_wait() when each waiter is woken up  
> and takes the futex.
>
> I apologize for the confusion on this.  Thanks for suggesting I send  
> this patch out independently from the rest Peter ;-)
>
> If we can come to a consensus on this, I suggest pulling this patch from  
> tip/core/futexes.

ok, i zapped it. The patches remaining are:

90621c4: futex: catch certain assymetric (get|put)_futex_key calls
42d35d4: futex: make futex_(get|put)_key() calls symmetric

	Ingo
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