linux-kernel - Re: [patch 14/19] softirq: Make softirq control and processing RT aware

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Message-ID: <20201123134437.GA95787@lothringen>
Date:   Mon, 23 Nov 2020 14:44:37 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        linux-um@...ts.infradead.org, Russell King <linux@...linux.org.uk>,
        Marc Zyngier <maz@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch 14/19] softirq: Make softirq control and processing RT
 aware

On Fri, Nov 13, 2020 at 03:02:21PM +0100, Thomas Gleixner wrote:
> +void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
> +{
> +	bool preempt_on = preemptible();
> +	unsigned long flags;
> +	u32 pending;
> +	int curcnt;
> +
> +	WARN_ON_ONCE(in_irq());
> +	lockdep_assert_irqs_enabled();
> +
> +	local_irq_save(flags);
> +	curcnt = this_cpu_read(softirq_ctrl.cnt);
> +
> +	/*
> +	 * If this is not reenabling soft interrupts, no point in trying to
> +	 * run pending ones.
> +	 */
> +	if (curcnt != cnt)
> +		goto out;
> +
> +	pending = local_softirq_pending();
> +	if (!pending || ksoftirqd_running(pending))
> +		goto out;
> +
> +	/*
> +	 * If this was called from non preemptible context, wake up the
> +	 * softirq daemon.
> +	 */
> +	if (!preempt_on) {
> +		wakeup_softirqd();
> +		goto out;
> +	}
> +
> +	/*
> +	 * Adjust softirq count to SOFTIRQ_OFFSET which makes
> +	 * in_serving_softirq() become true.
> +	 */
> +	cnt = SOFTIRQ_OFFSET;
> +	__local_bh_enable(cnt, false);

But then you enter __do_softirq() with softirq_count() == SOFTIRQ_OFFSET.
__do_softirq() calls softirq_handle_begin() which then sets it back to SOFTIRQ_DISABLE_OFFSET...

> +	__do_softirq();
> +
> +out:
> +	__local_bh_enable(cnt, preempt_on);

You escape from there with a correct preempt_count() but still the softirq executes
under SOFTIRQ_DISABLE_OFFSET and not SOFTIRQ_OFFSET, making in_serving_softirq() false.

> +	local_irq_restore(flags);
> +}
> +EXPORT_SYMBOL(__local_bh_enable_ip);

Thanks.