linux-kernel - Re: [PATCH RT 2/4] Revert "timers: do not raise softirq unconditionally"

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Message-ID: <20150326095302.2998cbe4@gandalf.local.home>
Date:	Thu, 26 Mar 2015 09:53:02 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>,
	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Carsten Emde <C.Emde@...dl.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Kacur <jkacur@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH RT 2/4] Revert "timers: do not raise softirq
 unconditionally"

On Thu, 26 Mar 2015 07:27:30 +0100
Mike Galbraith <umgwanakikbuti@...il.com> wrote:

> On Thu, 2015-03-26 at 06:23 +0100, Mike Galbraith wrote:
> 
> > I plan on taking a poke at getting "don't raise timer unconditionally"
> > working again when I get myself unburied, and see if I can come up with
> > a somewhat less icky way to work around take rtmutex in irq naughtiness.
> 
> Hm.. like maybe only do a fasttrylock with the wait lock already held
> via trylock, and don't bother turning it loose until we're done, to keep
> the sane people away.  That might work.. but may not be considered less
> icky by people equipped with that mysterious "taste" thingy ;-)

You would still need to add some ownership so that all will fail the
fast path.

You mean create a spin_trylock_in_hirq() which would just lock
the waitlock and not even do the fast path with the rt_mutex.

	if (!raw_spin_trylock(waitlock))
		goto failed_lock;

	if (!try_to_take_rt_mutex()) {
		raw_spin_unlock(waitlock);
		goto failed_lock;
	}

	return success;


With the waitlock held, no slow path will get to the pi code. Then you
have a spin_unlock_in_hirq() that would go right into the slow path
assuming the waitlock is already held.

Sounds reasonable to me.

-- Steve


	
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