linux-kernel - Re: [PATCH] futex: Handle transient "ownerless" rtmutex state correctly

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Message-ID: <87pn4tkuzn.fsf@nanos.tec.linutronix.de>
Date:   Wed, 04 Nov 2020 16:22:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mike Galbraith <efault@....de>,
        Gratian Crisan <gratian.crisan@...com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Brandon Streiff <brandon.streiff@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        James Minor <james.minor@...com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] futex: Handle transient "ownerless" rtmutex state correctly

On Wed, Nov 04 2020 at 16:12, Thomas Gleixner wrote:

> From: Mike Galbraith <efault@....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 ...