linux-kernel - Re: [PATCH v2 1/2] genirq: Update code comments wrt recycled thread

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Date:   Sun, 24 Jun 2018 11:47:31 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Lukas Wunner <lukas@...ner.de>
cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v2 1/2] genirq: Update code comments wrt recycled
 thread_mask

Lukas,

On Sun, 24 Jun 2018, Lukas Wunner wrote:

> Previously a race existed between __free_irq() and __setup_irq() wherein
> the thread_mask of a just removed action could be handed out to a newly
> added action and the freed irq thread would then tread on the oneshot
> mask bit of the newly added irq thread in irq_finalize_oneshot():
> 
> time
>  |  __free_irq()
>  |    raw_spin_lock_irqsave(&desc->lock, flags);
>  |    <remove action from linked list>
>  |    raw_spin_unlock_irqrestore(&desc->lock, flags);
>  |
>  |  __setup_irq()
>  |    raw_spin_lock_irqsave(&desc->lock, flags);
>  |    <traverse linked list to determine oneshot mask bit>
>  |    raw_spin_unlock_irqrestore(&desc->lock, flags);
>  |
>  |  irq_thread() of freed irq (__free_irq() waits in synchronize_irq())
>  |    irq_thread_fn()
>  |      irq_finalize_oneshot()
>  |        raw_spin_lock_irq(&desc->lock);
>  |        desc->threads_oneshot &= ~action->thread_mask;
>  |        raw_spin_unlock_irq(&desc->lock);
>  v
> 
> The race was known at least since 2012 when it was documented in a code
> comment by commit e04268b0effc ("genirq: Remove paranoid warnons and
> bogus fixups").

The race was known, but it was also harmless as nothing would touch stuff
after synchronize_irq().

> But it wasn't until 2017 that it was fixed by commit 9114014cf4e6
> ("genirq: Add mutex to irq desc to serialize request/free_irq()"),
> apparently inadvertantly so because the race is neither mentioned in the
> commit message nor was the code comment updated.  Make up for that.

Thanks for following up. This update is very well done.

       tglx