Re: [PATCH -v2] futex: fix reference leak

Darren Hart
2009-02-09T20:49:41Z

Peter Zijlstra wrote:
> On Mon, 2009-02-09 at 10:53 -0800, Pallipadi, Venkatesh wrote:
>> Yes. This patch fixes the problem on my system. With git, without the
>> patch I see the same oops as before on reboot. With the patch, system
>> reboots cleanly.
>>

OK, I remember why I didn't do this in the set I sent to Ingo for -tip. The unqueue_me()
call drops the key ref for us. Eventually I think we should take/release references and
locks in the same function as much as possible, but for now I left them where they were.
I suspect this patch is actually dropping one extra key ref than it should, but because
Caralin has been testing on linux.git and not -tip, the get/put patches I submitted there
aren't included, so this patch compensates for one of those missing patches :-)

Caralin, can you try applying the following patches from linux-tip without this patch and see if your problem still exists?

42d35d48ce7cefb9429880af19d1c329d1554e7a - futex: make futex_(get|put)_key() calls symmetric
90621c40cc4ab7b0a414311ce37e7cc7173403b6 - futex: catch certain assymetric (get|put)_futex_key calls

I haven't tried applying these to linux-2.6.git myself, so they may not apply cleanly.

Thanks,

Darren Hart

>> A minor typo in the patch
>> + ret = -RESTART_RESTARTBLOCK;
>> should be
>> + ret = -ERESTART_RESTARTBLOCK;
>
> Gah, so much for my copy-paste skillz ;-)
>
> Thanks!
>
> ---
> Catalin noticed that (38d47c1b7075: futex: rely on
> get_user_pages() for shared futexes) caused an mm_struct leak.
>
> Some tracing with the function graph tracer quickly pointed out that
> futex_wait() has exit paths with unbalanced reference counts.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Tested-by: "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>
> ---
> kernel/futex.c | 23 +++++++++++++++--------
> 1 files changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index f89d373..ff06c76 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1284,18 +1284,23 @@ retry:
> */
>
> /* If we were woken (and unqueued), we succeeded, whatever. */
> - if (!unqueue_me(&q))
> - return 0;
> - if (rem)
> - return -ETIMEDOUT;
> + if (!unqueue_me(&q)) {
> + ret = 0;
> + goto out_put_key;
> + }
> + if (rem) {
> + ret = -ETIMEDOUT;
> + goto out_put_key;
> + }
>
> /*
> * We expect signal_pending(current), but another thread may
> * have handled it for us already.
> */
> - if (!abs_time)
> - return -ERESTARTSYS;
> - else {
> + if (!abs_time) {
> + ret = -ERESTARTSYS;
> + goto out_put_key;
> + } else {
> struct restart_block *restart;
> restart = &current_thread_info()->restart_block;
> restart->fn = futex_wait_restart;
> @@ -1309,11 +1314,13 @@ retry:
> restart->futex.flags |= FLAGS_SHARED;
> if (clockrt)
> restart->futex.flags |= FLAGS_CLOCKRT;
> - return -ERESTART_RESTARTBLOCK;
> + ret = -ERESTART_RESTARTBLOCK;
> + goto out_put_key;
> }
>
> out_unlock_put_key:
> queue_unlock(&q, hb);
> +out_put_key:
> put_futex_key(fshared, &q.key);
>
> out:
>
>

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team