lkml.org 
[lkml]   [2009]   [Jan]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/2] futex: correct futex_requeue futex key ref counting in requeue loop

* Darren Hart <dvhltc@us.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@us.ibm.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Rusty Russell <rusty@au1.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


\
 
 \ /
  Last update: 2009-01-02 23:15    [W:0.121 / U:0.076 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site