linux-kernel - Re: [PATCH 3.14.25-rt22 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997!

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Date:	Mon, 23 Feb 2015 17:16:27 -0700
From:	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Thavatchai Makphaibulchoke <tmac@...com>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 3.14.25-rt22 1/2] rtmutex Real-Time Linux: Fixing kernel
 BUG at kernel/locking/rtmutex.c:997!

On 02/23/2015 11:37 AM, Steven Rostedt wrote:
> 
> OK, I believe I understand the issue. Perhaps it would be much better
> to create a fake task per CPU that we use when grabbing locks in
> interrupt mode. And make these have a priority of 0 (highest), since
> they can not be preempted, they do have such a priority.
> 
> Then in the fast trylock and unlock code, we can add:
> 
> 	struct task_struct *curr = current;
> 
> 	if (unlikely(in_irq()))
> 		curr = this_cpu_read(irq_task);
> 
> This way the priority inheritance will stop when it hits this task (no
> need to boost a task of highest priority), and we can leave that code
> alone.
> 

Thanks again for the comments and suggestion.

Yes, creating a per cpu fake task was one of the alternative considered.
 I believe one of the reasons I did not purse is the amount of extra
storage it requires (sizeof(struct task_struct) * number of cpus.
Though the changes may not be as intrusive as the one I sent, some are
still required, mainly with current (one in particular came to mind is
in wakeup_next-watier()).

If I'm not mistaken, another reason could also be due to the rate of the
timer interrupt, in the case that the mutex is highly contested IH could
stall the non-real-time requester for a long time, even to the point of
the cpu is perceived as hung.

Anyway, I'll retry the fake task approach a try and report back if there
is any issue.

Thanks,
Mak.

> -- Steve
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