linux-kernel - Re: [PATCH RT] locking/rtmutex: don't drop the wait

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Message-ID: <20170921135135.19a84dd1@gandalf.local.home>
Date:   Thu, 21 Sep 2017 13:51:35 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de
Subject: Re: [PATCH RT] locking/rtmutex: don't drop the wait_lock twice

On Thu, 21 Sep 2017 19:35:12 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> No, it does not. It fixes only one and this one was introduced while I
> rebased RT ontop of the futex work - the patch "futex: Fix bug on when a
> requeued RT task times out" to be exact.
> If you look at the code in the v4.9 or v4.11 RT then you see that there
> is just rt_mutex_start_proxy_lock()() and this function acquires and
> releases ->wait_lock lock. After the futex rework the locking changed
> and I missed to adapt the RT-only patch I mentioned.
> Again: without the patch, the ->wait_lock is dropped twice in the error
> case here: once here and the second time by the caller and this has only
> been like this since the futex-rework. So this does not apply to v4.1-RT
> for instance because the futex rework got into v4.9.18-rt14 and I don't
> recall that you backported it.

Looking at the code, you are correct that it only fixes one bug. But I
would still include a statement about why there's a change in semantics
here (there still is, the old way enabled interrupts before returning,
this way does not). The reason this is OK, is because the
spin_unlock_irq() is called immediately after this function.

-- Steve