Reply-To: xlpang@redhat.com
Subject: Re: [PATCH] sched/deadline/rtmutex: Fix a PI crash for deadline tasks
References: <1459508418-25577-1-git-send-email-xlpang@redhat.com>
 <20160401113827.GQ3430@twins.programming.kicks-ass.net>
 <56FE685E.6080001@redhat.com>
 <19912883-8AB1-4DFD-A0E1-F23057785243@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>, xlpang@redhat.com
Cc: linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@arm.com>,
        Ingo Molnar <mingo@redhat.com>, Steven Rostedt <rostedt@goodmis.org>
From: Xunlei Pang <xpang@redhat.com>
Message-ID: <56FE78E0.5060504@redhat.com>
Date: Fri, 1 Apr 2016 21:34:24 +0800
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On 2016/04/01 at 21:12, Peter Zijlstra wrote:
>
> On 1 April 2016 14:23:58 CEST, Xunlei Pang <xpang@redhat.com> wrote:
>
>>>> We need this iff lock owner has the deadline priority.
>>> How is this deadline specific, those functions you modify are
>>> deadline/rt agnostic.
>> I checked the code, currently only deadline accesses the
>> pi_waiters/pi_waiters_leftmost
>> without pi_lock held via rt_mutex_get_top_task(), other cases all have
>> pi_lock held.
>>
>> So adding the condition.
> How does that not suggest fixing the deadline code?

I did tried that at first, but found it very hard when processing
pull_dl_task(push_dl_task can crash as well) like:
   double_lock_balance(this_rq, src_rq);
   p = pick_earliest_pushable_dl_task(src_rq, this_cpu);
  /*  and for each @p, we must hold its pi_lock,
       doing this once rq is locked will cause deadlock. */

Ditto for enqueue_task_dl()->rt_mutex_get_top_task(), as rq is locked.

If we unlock rq first and then lock pi_lock, this may cause other problems
due to unlocking rq.

Any better ideas is welcome.

Regards,
Xunlei