linux-kernel - Re: WARN_ON_ONCE(!new_owner) within wake_futex

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Message-ID: <20190130210733.mg6aascw2gzl3oqz@linutronix.de>
Date:   Wed, 30 Jan 2019 22:07:33 +0100
From:   Sebastian Sewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-s390@...r.kernel.org,
        Stefan Liebler <stli@...ux.ibm.com>
Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered

On 2019-01-30 18:56:54 [+0100], Thomas Gleixner wrote:
> TBH, no clue. Below are some more traceprintks which hopefully shed some
> light on that mystery. See kernel/futex.c line 30  ...

The robust list it somehow buggy. In the last trace we had the
handle_futex_death() of uaddr 3ff9e880140 as the last action. That means
it was an entry in 56496's ->list_op_pending entry. This makes sense
because it tried to acquire the lock, failed, got killed.

According to uaddr pid 56956 is the owner. So 56956 invoked one of
pthread_mutex_lock() / pthread_mutex_timedlock() /
pthread_mutex_trylock() and should have obtained the lock in userland.
Depending on where it got killed, that mutex should be either recorded
in ->list_op_pending or the robust_list (or both if it didn't clear
->list_op_pending yet). But it is not.
Similar for pthread_mutex_unlock().
We don't have a trace_point if we abort processing the list.
On the other hand, it didn't trigger on x86 for hours. Could the atomic
ops be the culprit?

> Thanks,
> 
> 	tglx

Sebastian