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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Mike Galbraith <efault@gmx.de>,
        Gratian Crisan <gratian.crisan@ni.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Brandon Streiff <brandon.streiff@ni.com>,
        Ingo Molnar <mingo@redhat.com>,
        Darren Hart <dvhart@infradead.org>,
        James Minor <james.minor@ni.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH] futex: Handle transient "ownerless" rtmutex state correctly
In-Reply-To: <87sg9pkvf7.fsf@nanos.tec.linutronix.de>
References: <87a6w6x7bb.fsf@ni.com> <878sbixbk4.fsf@ni.com> <2376f4e71c638aee215a4911e5efed14c5adf56e.camel@gmx.de> <5f536491708682fc3b86cb5b7bc1e05ffa3521e7.camel@gmx.de> <874km5mnbf.fsf@nanos.tec.linutronix.de> <871rh9mkvr.fsf@nanos.tec.linutronix.de> <87v9ell0cn.fsf@nanos.tec.linutronix.de> <a9e88887c027b11596cd7fb96c425011c36e5d29.camel@gmx.de> <87sg9pkvf7.fsf@nanos.tec.linutronix.de>
Date:   Wed, 04 Nov 2020 16:22:04 +0100
Message-ID: <87pn4tkuzn.fsf@nanos.tec.linutronix.de>
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On Wed, Nov 04 2020 at 16:12, Thomas Gleixner wrote:

> From: Mike Galbraith <efault@gmx.de>
>
> Gratian managed to trigger the BUG_ON(!newowner) in fixup_pi_state_owner().
> This is one possible chain of events leading to this:
>
> Task Prio       Operation
> T1   120	lock(F)
> T2   120	lock(F)   -> blocks (top waiter)
> T3   50 (RT)	lock(F)   -> boosts T3 and blocks (new top waiter)

boosts T1 obviously as Sebastian just pointed out to me. /me pulls the
futex induced brain damage excuse ...