linux-kernel - Re: [PATCH 2/3] locking: ww_mutex: Allow to use rt

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Message-ID: <54FF3616.8060004@canonical.com>
Date:	Tue, 10 Mar 2015 19:21:10 +0100
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH 2/3] locking: ww_mutex: Allow to use rt_mutex instead
 of mutex for the baselock

Hey,

Op 10-03-15 om 16:28 schreef Peter Zijlstra:
> On Tue, Mar 10, 2015 at 03:10:46PM +0100, Maarten Lankhorst wrote:
>> Op 10-03-15 om 13:37 schreef Peter Zijlstra:
> 
>>> So IIRC this is the function that checks who gets wounded (and gets to
>>> do the whole retry thing), right?
>>>
>>> So for the RT case, I think we should extend it to not (primarily) be a
>>> FIFO thing, but also consider the priority of the tasks involved.
>>>
>>> Maybe a little something like:
>>>
>>> 	if (hold_ctx->task->prio < ctx->task->prio)
>>> 		return -EDEADLOCK;
>>>
>>> before the timestamp check; although I suppose we should also add a
>>> deadline test in case both prios are -1.
> 
>> I think that's useful but if you implement -EDEADLK based on thread
>> priority, any boosted thread should receive -EDEADLK when it tries to
>> acquire a new lock in the same context, to force it to back off..
> 
> But _only_ if the boost comes from inside the ww_ctx, right?
> 
> Yeah, implementing that might be tricky, maybe we can keep it as a note
> for now ;)
Indeed!

~Maarten
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