linux-kernel - Re: [PATCH] sched/deadline/rtmutex: Fix a PI crash for deadline tasks

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Message-ID: <570A0D37.8080400@redhat.com>
Date:	Sun, 10 Apr 2016 16:22:15 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Juri Lelli <juri.lelli@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/deadline/rtmutex: Fix a PI crash for deadline tasks

On 2016/04/09 at 21:29, Peter Zijlstra wrote:
> On Sat, Apr 09, 2016 at 11:25:39AM +0800, Xunlei Pang wrote:
>
>>> In any case, I just realized we do not in fact provide this guarantee
>>> (of pointing to a blocked task) that needs a bit more work.
>> Current patch calls rt_mutex_adjust_prio() before wake_up_q() the
>> wakee, at that moment the wakee has been removed by
>> rt_mutex_slowunlock()->mark_wakeup_next_waiter(), from current's
>> pi_waiters, rt_mutex_adjust_prio() won't see this wakee, so I think
>> this should not be problem.
> No, any wakeup after mark_wakeup_next_waiter() will make the task run.
> And since we must always consider spurious wakeups, we cannot rely on us
> (eg. our wake_up_q call) being the one and only.
>
> Therefore it is possible and the only thing that stands between us and
> doom is the fact that the wake_q stuff holds a task reference.
>
> But we cannot guarantee that the task we have a pointer to is in fact
> blocked.

Does that really matters? the pointer is accessed on behalf of current,  and current
will call rt_mutex_adjust_prio() very soon to update the right pointer.

Also the pointer is used to update current's deadline/runtime, we can restore these
params in rt_mutex_setprio() for deboost cases. I just checked current code,  it did
nothing to restore current's deadline/runtime when deboosting, maybe we can leave
this job to future deadline bandwidth inheritance?

Regards,
Xunlei