linux-kernel - Re: [PATCH v2 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997!

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Date:	Tue, 7 Apr 2015 11:12:09 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thavatchai Makphaibulchoke <tmac@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Jason Low <jason.low2@...com>
Subject: Re: [PATCH v2 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997!

On Tue, Apr 7, 2015 at 5:04 AM, Peter Zijlstra <peterz@...radead.org> wrote:

> That smells like something we should be able to do without a lock.
>
> If we use {READ,WRITE}_ONCE() on those two fields (->active_timers and
> ->next_timer) we should be able to do this without the spinlock.

Yeah, when atomics were suggested earlier, I was wondering if we could
just use READ_ONCE/WRITE_ONCE.

> Races here aren't really a problem I think, if you manage to install a
> timer at the current jiffy and have already missed the tick you're in
> the same boat. You get to wait for the next tick.

The lock shouldn't be used in get_next_timer_interrupt() either right?

unsigned long get_next_timer_interrupt(unsigned long now)
{
...

#ifdef CONFIG_PREEMPT_RT_FULL
        /*
         * On PREEMPT_RT we cannot sleep here. If the trylock does not
         * succeed then we return the worst-case 'expires in 1 tick'
         * value.  We use the rt functions here directly to avoid a
         * migrate_disable() call.
         */
        if (!spin_do_trylock(&base->lock))
                return  now + 1;
#else
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