linux-kernel - Re: [PATCH] irq_poll: Use raise_softirq_irqoff() in cpu

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Message-ID: <YW1SIE08f3X3joxe@infradead.org>
Date:   Mon, 18 Oct 2021 03:53:20 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] irq_poll: Use raise_softirq_irqoff() in cpu_dead notifier

On Thu, Sep 30, 2021 at 12:37:54PM +0200, Sebastian Andrzej Siewior wrote:
> __raise_softirq_irqoff() adds a bit to the pending sofirq mask and this
> is it. The softirq won't be handled in a deterministic way but randomly
> when an interrupt fires and handles softirq in its irq_exit() routine or
> if something randomly checks and handles pending softirqs in the call
> chain before the CPU goes idle.
> 
> Add a local_bh_disable/enable() around the IRQ-off section which will
> handle pending softirqs.

This patch leaves me extremely confused, and it would even more if I was
just reading the code.  local_irq_disable is supposed to disable BHs
as well, so the code looks pretty much nonsensical to me.  But
apparently that isn't the point if I follow your commit message as you
don't care about an extra level of BH disabling but want to force a
side-effect of the re-enabling?  Why not directly call the helper
to schedule the softirq then?