linux-kernel - Re: [PATCH 2/2] rtmutex: Reduce top-waiter blocking on a lock

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Date:   Tue, 17 Apr 2018 09:52:10 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     kbuild test robot <lkp@...el.com>
Cc:     kbuild-all@...org, peterz@...radead.org, tglx@...utronix.de,
        mingo@...nel.org, longman@...hat.com, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbues@...e.de>
Subject: Re: [PATCH 2/2] rtmutex: Reduce top-waiter blocking on a lock

On Wed, 11 Apr 2018, kbuild test robot wrote:

>Hi Davidlohr,
>
>I love your patch! Perhaps something to improve:
>
>[auto build test WARNING on linus/master]
>[also build test WARNING on v4.16 next-20180411]
>[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>
>url:    https://github.com/0day-ci/linux/commits/Davidlohr-Bueso/locking-rtmutex-Delete-save_state-member-of-struct-rt_mutex/20180411-155733
>reproduce:
>        # apt-get install sparse
>        make ARCH=x86_64 allmodconfig
>        make C=1 CF=-D__CHECK_ENDIAN__

Ok.

>
>
>sparse warnings: (new ones prefixed by >>)
>
>>> kernel/locking/rtmutex_common.h:62:9: sparse: context imbalance in '__rt_mutex_slowlock' - unexpected unlock

I did:

make C=1 CF=-D__CHECK_ENDIAN__ kernel/locking/rtmutex.o

But cannot trigger this message. There are however a ton of other
sparse issues with 'indirect_branch'. Ie:

./arch/x86/include/asm/nospec-branch.h:144:38: warning: Unknown escape '@'
./include/linux/init.h:134:6: error: attribute 'indirect_branch': unknown attribute
./include/linux/init.h:135:5: error: attribute 'indirect_branch': unknown attribute

More importantly, I don't follow where this "unexpected unlock" would come from
considering that rtmutex doesn't use __acquires/__releases annotations. Nor do
I understand why this patch would produce a new context imbalance.

Thanks,
Davidlohr