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Date:   Tue, 14 Sep 2021 13:09:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jason Wang <jasowang@...hat.com>, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, f.hetzelt@...berlin.de,
        david.kaplan@....com, konrad.wilk@...cle.com,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH 7/9] virtio-pci: harden INTX interrupts

On Tue, Sep 14 2021 at 13:03, Peter Zijlstra wrote:
> On Mon, Sep 13, 2021 at 11:36:24PM +0200, Thomas Gleixner wrote:
> Here you rely on the UNLOCK+LOCK pattern because we have two adjacent
> critical sections (or rather, the same twice), which provides RCtso
> ordering, which is sufficient to make the below store:
>
>> 
>>         intx_soft_enabled = true;
>
> a RELEASE. still, I would suggest writing it at least using
> WRITE_ONCE() with a comment on.

Right. forgot about that.

> 	disable_irq();
> 	/*
> 	 * The above disable_irq() provides TSO ordering and as such
> 	 * promotes the below store to store-release.
> 	 */
> 	WRITE_ONCE(intx_soft_enabled, true);
> 	enable_irq();
>
>> In this case synchronize_irq() prevents the subsequent store to
>> intx_soft_enabled to leak into the __disable_irq(desc) section which in
>> turn makes it impossible for an interrupt handler to observe
>> intx_soft_enabled == true before the prerequisites which preceed the
>> call to disable_irq() are visible.
>> 
>> Of course the memory ordering wizards might disagree, but if they do,
>> then we have a massive chase of ordering problems vs. similar constructs
>> all over the tree ahead of us.
>
> Your case, UNLOCK s + LOCK s, is fully documented to provide RCtso
> ordering. The more general case of: UNLOCK r + LOCK s, will shortly
> appear in documentation near you. Meaning we can forget about the
> details an blanket state that any UNLOCK followed by a LOCK (on the same
> CPU) will provide TSO ordering.

I think we also should document the disable/synchronize_irq() scheme
somewhere.

Thanks,

        tglx

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