linux-kernel - Re: [PATCH 4.4 3/3] futex: fix dead code in attach_to_pi

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Date:   Wed, 10 Mar 2021 13:28:02 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Zheng Yejian <zhengyejian1@...wei.com>, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        cj.chengjian@...wei.com, judy.chenhui@...wei.com,
        zhangjinhao2@...wei.com, nixiaoming@...wei.com
Subject: Re: [PATCH 4.4 3/3] futex: fix dead code in attach_to_pi_owner()

On Wed, 10 Mar 2021, Greg KH wrote:

> On Tue, Mar 09, 2021 at 06:14:37PM +0000, Lee Jones wrote:
> > On Tue, 09 Mar 2021, Greg KH wrote:
> > 
> > > On Tue, Mar 09, 2021 at 11:06:05AM +0800, Zheng Yejian wrote:
> > > > From: Thomas Gleixner <tglx@...utronix.de>
> > > > 
> > > > The handle_exit_race() function is defined in commit 9c3f39860367
> > > >  ("futex: Cure exit race"), which never returns -EBUSY. This results
> > > > in a small piece of dead code in the attach_to_pi_owner() function:
> > > > 
> > > > 	int ret = handle_exit_race(uaddr, uval, p); /* Never return -EBUSY */
> > > > 	...
> > > > 	if (ret == -EBUSY)
> > > > 		*exiting = p; /* dead code */
> > > > 
> > > > The return value -EBUSY is added to handle_exit_race() in upsteam
> > > > commit ac31c7ff8624409 ("futex: Provide distinct return value when
> > > > owner is exiting"). This commit was incorporated into v4.9.255, before
> > > > the function handle_exit_race() was introduced, whitout Modify
> > > > handle_exit_race().
> > > > 
> > > > To fix dead code, extract the change of handle_exit_race() from
> > > > commit ac31c7ff8624409 ("futex: Provide distinct return value when owner
> > > >  is exiting"), re-incorporated.
> > > > 
> > > > Lee writes:
> > > > 
> > > > This commit takes the remaining functional snippet of:
> > > > 
> > > >  ac31c7ff8624409 ("futex: Provide distinct return value when owner is exiting")
> > > > 
> > > > ... and is the correct fix for this issue.
> > > > 
> > > > Fixes: 9c3f39860367 ("futex: Cure exit race")
> > > > Cc: stable@...r.kernel.org # v4.9.258
> > > > Signed-off-by: Xiaoming Ni <nixiaoming@...wei.com>
> > > > Reviewed-by: Lee Jones <lee.jones@...aro.org>
> > > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > Signed-off-by: Zheng Yejian <zhengyejian1@...wei.com>
> > > > ---
> > > >  kernel/futex.c | 6 +++---
> > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > Same here, what is the upstream git id?
> > 
> > It doesn't have one as such - it's a part-patch:
> > 
> > > > This commit takes the remaining functional snippet of:
> > > > 
> > > >  ac31c7ff8624409 ("futex: Provide distinct return value when owner is exiting")
> 
> That wasn't obvious :(

This was also my thinking, which is why I replied to the original
patch in an attempt to clarify what I thought was happening.

> Is this a backport of another patch in the stable tree somewhere?

Yes, it looks like it.

The full patch was back-ported to v4.14 as:

  e6e00df182908f34360c3c9f2d13cc719362e9c0

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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