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

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Message-ID: <20150324181034.GD1321@linutronix.de>
Date:	Tue, 24 Mar 2015 19:10:34 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.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>,
	John Kacur <jkacur@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH RT 2/4] Revert "timers: do not raise softirq
 unconditionally"

* Steven Rostedt | 2015-03-19 12:26:11 [-0400]:

>On Thu, 19 Mar 2015 09:17:09 +0100
>Mike Galbraith <umgwanakikbuti@...il.com> wrote:
>
>
>> (aw crap, let's go shopping)... so why is the one in timer.c ok?
>
>It's not. Sebastian, you said there were no other cases of rt_mutexes
>being taken in hard irq context. Looks like timer.c has one.

If you refer to switch_timer_base() then this one is not taken in
hard-irq context. The callchain is:

lock_timer_base() (with spin_lock_irqsave(&base->lock, *flags) which
                   makes it a sleeping lock or lockdep would scream)
  -> switch_timer_base()
     -> spin_trylock() (not in hardirq conteyt)

>So perhaps the real fix is to get that special case of ownership in
>hard interrupt context?

I'm really not sure we want to keep doing this.

>
>-- Steve

Sebastian
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