linux-kernel - Re: [PATCH] kernel/futex.c: notice the return value after rt_mutex_finish_proxy

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Date:	Thu, 12 Sep 2013 16:32:00 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chen Gang <gang.chen@...anux.com>
cc:	dvhart@...ux.intel.com, ccross@...roid.com,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/futex.c: notice the return value after
 rt_mutex_finish_proxy_lock() fails

On Tue, 20 Aug 2013, Chen Gang wrote:

> rt_mutex_finish_proxy_lock() can return failure code (e.g. -EINTR,
> -ETIMEDOUT).
> 
> Original implementation has already noticed about it, but not check it
> before next work.
> 
> Also let coments within 80 columns to pass "./scripts/checkpatch.pl".
> 
> 
> Signed-off-by: Chen Gang <gang.chen@...anux.com>
> ---
>  kernel/futex.c |   30 ++++++++++++++++--------------
>  1 files changed, 16 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index c3a1a55..1a94e7d 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2373,21 +2373,23 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
>  		ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1);
>  		debug_rt_mutex_free_waiter(&rt_waiter);
>  
> -		spin_lock(q.lock_ptr);
> -		/*
> -		 * Fixup the pi_state owner and possibly acquire the lock if we
> -		 * haven't already.
> -		 */
> -		res = fixup_owner(uaddr2, &q, !ret);
> -		/*
> -		 * If fixup_owner() returned an error, proprogate that.  If it
> -		 * acquired the lock, clear -ETIMEDOUT or -EINTR.
> -		 */
> -		if (res)
> -			ret = (res < 0) ? res : 0;
> +		if (!ret) {

Again. This is completely wrong!  

We MUST call fixup_owner even if finish_proxy_lock() returned with an
error code. Simply because finish_proxy_lock() is called outside of
the spin_lock(q.lock_ptr) region and another thread might have
modified the futex state. So we need to handle the corner cases
otherwise we might leave the futex in some undefined state.

You're reintroducing a hard to decode bug, which got analyzed and
fixed in futex_lock_pi() years ago. See the history for the
explanation.

Sigh.

	tglx



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