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Date:   Sun, 12 Feb 2017 14:23:46 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     linux-kernel@...r.kernel.org
Cc:     intel-gfx@...ts.freedesktop.org,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Hannes Reinecke <hare@...e.com>, Jens Axboe <axboe@...nel.dk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chen Fan <chen.fan.fnst@...fujitsu.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Emese Revfy <re.emese@...il.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Eric Dumazet <edumazet@...gle.com>,
        Tom Herbert <therbert@...gle.com>,
        Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH] softirq: Prevent looping on disabled tasklets

On Sun, Feb 12, 2017 at 02:00:19PM +0000, Chris Wilson wrote:
> Disabling a tasklet causes it not to run during tasklet_action, but is
> put back onto the runnable tasklet list, and a new softirq raised. As
> the softirq is raised from within __do_softirq() this causing
> __do_softirq() to loop constantly until its timeslice expires and is
> transferred to the ksoftirq thread. ksoftirq then permanently spins,
> as on each action, the disabled tasklet keeps reraising the softirq.
> 
> Break this vicious cycle by moving the softirq from the action to the
> final tasklet_enable().
> 
> This behaviour appears to be historic (since the first git import).
> However, the looping until timeslice duration (to a max of 2ms) was
> first introduced in commit c10d73671ad3 ("softirq: reduce latencies"),
> with the restart limit restored in commit 34376a50fb1f ("Fix lockup
> related to stop_machine being stuck in __do_softirq.")
> 
> Reported-by: Tvrtko Ursulin <tvrtko.ursulin@...el.com>
> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Hannes Reinecke <hare@...e.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: Alexander Potapenko <glider@...gle.com>
> Cc: Chen Fan <chen.fan.fnst@...fujitsu.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Johannes Thumshirn <jthumshirn@...e.de>
> Cc: Emese Revfy <re.emese@...il.com>
> Cc: Sagi Grimberg <sagi@...mberg.me>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Tom Herbert <therbert@...gle.com>
> Cc: Ben Hutchings <bhutchings@...arflare.com>
> ---
>  include/linux/interrupt.h | 7 +++++--
>  kernel/softirq.c          | 2 --
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index 53144e78a369..12750f00d00d 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -613,8 +613,11 @@ static inline void tasklet_disable(struct tasklet_struct *t)
>  
>  static inline void tasklet_enable(struct tasklet_struct *t)
>  {
> -	smp_mb__before_atomic();
> -	atomic_dec(&t->count);
> +	if (!atomic_dec_and_test(&t->count))
> +		return;
> +
> +	if (test_bit(TASKLET_STATE_SCHED, &t->state))
> +		raise_softirq(HI_SOFTIRQ | TASKLET_SOFTIRQ);

raise_softirq is not exported, so let's move tasklet_enable()
out-of-line and export that.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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