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Date:   Mon, 10 Apr 2023 11:05:36 +0100
Message-ID: <87fs9859xr.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Shanker Donthineni <sdonthineni@nvidia.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Michael Walle <michael@walle.cc>, linux-kernel@vger.kernel.org,
        Vikram Sethi <vsethi@nvidia.com>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>
Subject: Re: [PATCH v2 1/3] genirq: Use hlist for managing resend handlers
In-Reply-To: <76bc7b50-d47f-e5a7-6aa6-54a7b1492ea3@nvidia.com>
References: <20230408171526.3059039-1-sdonthineni@nvidia.com>
        <20230408171526.3059039-2-sdonthineni@nvidia.com>
        <863559mne0.wl-maz@kernel.org>
        <76bc7b50-d47f-e5a7-6aa6-54a7b1492ea3@nvidia.com>
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On Sun, 09 Apr 2023 13:00:27 +0100,
Shanker Donthineni <sdonthineni@nvidia.com> wrote:
> 
> >> @@ -30,18 +31,17 @@ static DECLARE_BITMAP(irqs_resend, IRQ_BITMAP_BITS);
> >>   static void resend_irqs(struct tasklet_struct *unused)
> >>   {
> >>        struct irq_desc *desc;
> >> -     int irq;
> >> -
> >> -     while (!bitmap_empty(irqs_resend, nr_irqs)) {
> >> -             irq = find_first_bit(irqs_resend, nr_irqs);
> >> -             clear_bit(irq, irqs_resend);
> >> -             desc = irq_to_desc(irq);
> >> -             if (!desc)
> >> -                     continue;
> >> -             local_irq_disable();
> >> +
> >> +     raw_spin_lock_irq(&irq_resend_lock);
> >> +     while (!hlist_empty(&irq_resend_list)) {
> >> +             desc = hlist_entry(irq_resend_list.first, struct irq_desc,
> >> +                                resend_node);
> >> +             hlist_del_init(&desc->resend_node);
> >> +             raw_spin_unlock(&irq_resend_lock);
> >>                desc->handle_irq(desc);
> >> -             local_irq_enable();
> >> +             raw_spin_lock(&irq_resend_lock);
> > 
> > What makes it safe to drop the local_irq_*able()?
> > 
> > tasklet_action_common() explicitly enables interrupts when calling the
> > callback, so unless there is some other interrupt disabling that I
> > can't immediately spot, the handler may run in the wrong context.
> > 
> 
> Unless I am overlooking something, interrupts are disabled within the while
> loop unless desc->handle_irq() is enabling it. The existing code disables
> and enables interrupts for each handler invocation, whereas the modified
> code does it only once for all outstanding handlers.

Ah, you use raw_spinlock_irq() outside of the loop. I somehow glanced
over that, apologies for the noise. Unless we expect a really long
list of interrupts to be resent, your current code should be OK.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.