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Date: Tue, 19 Dec 2017 09:58:44 +0100
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Christoph Hellwig <hch@lst.org>, keescook@chromium.org,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 17/25] hrtimer: Implementation of softirq hrtimer handling
Message-ID: <20171219085843.l55fasrfdqdyta5z@breakpoint.cc>
References: <20170831105725.809317030@linutronix.de>
 <20170831105826.921969670@linutronix.de>
 <20170927164025.GI17526@worktop.programming.kicks-ass.net>
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this is late I know…

On 2017-09-27 18:40:26 [+0200], Peter Zijlstra wrote:
>  - removed superfluous local_bh_disable(), since local_irq_disable()
>    already implies much the same.

it is not superfluous.

> Please consider...
> 
> @@ -1768,7 +1786,6 @@ int hrtimers_dead_cpu(unsigned int scpu)
>  	BUG_ON(cpu_online(scpu));
>  	tick_cancel_sched_timer(scpu);
>  
> -	local_bh_disable();
>  	local_irq_disable();
>  	old_base = &per_cpu(hrtimer_bases, scpu);
>  	new_base = this_cpu_ptr(&hrtimer_bases);
> @@ -1796,7 +1813,6 @@ int hrtimers_dead_cpu(unsigned int scpu)
>  	/* Check, if we got expired work to do */
>  	__hrtimer_peek_ahead_timers();
>  	local_irq_enable();
> -	local_bh_enable();
>  	return 0;
>  }

we need in there. That local_bh_disable() is required in order to let
raise_softirq_irqoff() not do anything stupid in particular we need
!in_interrupt() defer wakeup_softirqd() until local_bh_enable().
Otherwise wakeup_softirqd() might actually try to wakeup the process and
go after the pi_lock which can't happen while holding cpu_base->lock.

Sebastian