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Date: Wed, 6 Apr 2016 20:14:33 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Xunlei Pang <xlpang@redhat.com>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Juri Lelli <juri.lelli@arm.com>, Ingo Molnar <mingo@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2 1/2] sched/rtmutex/deadline: Fix a PI crash for
 deadline tasks
Message-ID: <20160406181433.GT3448@twins.programming.kicks-ass.net>
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On Wed, Apr 06, 2016 at 08:59:15PM +0800, Xunlei Pang wrote:
> A crash happened while I'm playing with deadline PI rtmutex.
> 
>     BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
>     IP: [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30
>     PGD 232a75067 PUD 230947067 PMD 0
>     Oops: 0000 [#1] SMP
>     CPU: 1 PID: 10994 Comm: a.out Not tainted
> 
>     Call Trace:
>     [<ffffffff810cf8aa>] ? enqueue_task_dl+0x2a/0x320
>     [<ffffffff810b658c>] enqueue_task+0x2c/0x80
>     [<ffffffff810ba763>] activate_task+0x23/0x30
>     [<ffffffff810d0ab5>] pull_dl_task+0x1d5/0x260
>     [<ffffffff810d0be6>] pre_schedule_dl+0x16/0x20
>     [<ffffffff8164e783>] __schedule+0xd3/0x900
>     [<ffffffff8164efd9>] schedule+0x29/0x70
>     [<ffffffff8165035b>] __rt_mutex_slowlock+0x4b/0xc0
>     [<ffffffff81650501>] rt_mutex_slowlock+0xd1/0x190
>     [<ffffffff810eeb33>] rt_mutex_timed_lock+0x53/0x60
>     [<ffffffff810ecbfc>] futex_lock_pi.isra.18+0x28c/0x390
>     [<ffffffff810cfa15>] ? enqueue_task_dl+0x195/0x320
>     [<ffffffff810d0bac>] ? prio_changed_dl+0x6c/0x90
>     [<ffffffff810ed8b0>] do_futex+0x190/0x5b0
>     [<ffffffff810edd50>] SyS_futex+0x80/0x180
>     [<ffffffff8165a089>] system_call_fastpath+0x16/0x1b
>     RIP  [<ffffffff810eeb8f>] rt_mutex_get_top_task+0x1f/0x30
> 
> This is because rt_mutex_enqueue_pi() and rt_mutex_dequeue_pi()
> are only protected by pi_lock when operating pi waiters, while
> rt_mutex_get_top_task() will access them with rq lock held but
> not holding pi_lock.
> 
> In order to tackle it, we introduce a new pointer "pi_top_task"
> in task_struct, and update it to be the top waiter task(this waiter
> is updated under pi_lock) in rt_mutex_setprio() which is under 
> both pi_lock and rq lock, then ensure all its accessers be under 
> rq lock (or pi_lock), this can safely fix the crash.
> 
> This patch is originated from "Peter Zijlstra", with several
> tweaks and improvements by me.

I would suggest doing the rt_mutex_postunlock() thing as a separate
patch, it has some merit outside of these changes and reduces the total
amount of complexity in this patch.

Also, I would very much like Thomas to ack this patch before I take it,
but since its conference season this might take a little while. Esp. the
change marked with XXX is something that I'm not sure about.