linux-kernel - Re: [RFC RESEND v10 03/14] irq & spin_lock: Add counted interrupt disabling/enabling

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Message-ID: <20250617101120.7c946656@gandalf.local.home>
Date: Tue, 17 Jun 2025 10:11:20 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org, Daniel
 Almeida <daniel.almeida@...labora.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
 <dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, Mel Gorman
 <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Will Deacon
 <will@...nel.org>, Waiman Long <longman@...hat.com>, Miguel Ojeda
 <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, Andreas
 Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor
 Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, David
 Woodhouse <dwmw@...zon.co.uk>, Jens Axboe <axboe@...nel.dk>, Sebastian
 Andrzej Siewior <bigeasy@...utronix.de>, NeilBrown <neilb@...e.de>, Caleb
 Sander Mateos <csander@...estorage.com>, Ryo Takakura
 <ryotkkr98@...il.com>, K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [RFC RESEND v10 03/14] irq & spin_lock: Add counted interrupt
 disabling/enabling

On Tue, 27 May 2025 18:21:44 -0400
Lyude Paul <lyude@...hat.com> wrote:

> +static inline void local_interrupt_enable(void)
> +{
> +	int new_count;
> +
> +	new_count = hardirq_disable_exit();
> +
> +	if ((new_count & HARDIRQ_DISABLE_MASK) == 0) {
> +		unsigned long flags;
> +
> +		flags = raw_cpu_read(local_interrupt_disable_state.flags);
> +		local_irq_restore(flags);
> +		/*
> +		 * TODO: re-read preempt count can be avoided, but it needs
> +		 * should_resched() taking another parameter as the current
> +		 * preempt count
> +		 */
> +#ifdef PREEMPTION
> +		if (should_resched(0))
> +			__preempt_schedule();
> +#endif
> +	}
> +}

I'm confused to why the should_resched() is needed? We are handling
interrupts right? The hardirq_disable_exit() will set preempt_count to zero
before we enable interrupts. When the local_irq_restore() enables interrupts
again, if there's an interrupt pending it will trigger then. If the
interrupt sets NEED_RESCHED, when it returns from the interrupt handler, it
will see preempt_count as zero, right?

If it does, then it will call schedule before it gets back to this code.

-- Steve