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

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Date:   Thu, 21 Sep 2017 20:16:15 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
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 2017-09-21 13:51:35 [-0400], Steven Rostedt wrote:
> 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.

The spin_lock_irq() was never called in this function. I will update it.

> -- Steve

Sebastian