linux-kernel - Re: [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu affinity mask

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Date:   Tue, 24 Oct 2023 18:53:28 +0800
From:   Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-team@...udflare.com, Caleb Raitto <caraitto@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu affinity mask

On Tue, 24 Oct 2023 10:17:19 +0200, Jakub Sitnicki <jakub@...udflare.com> wrote:
> On Tue, Oct 24, 2023 at 10:31 AM +08, Xuan Zhuo wrote:
> > On Mon, 23 Oct 2023 18:52:45 +0200, Jakub Sitnicki <jakub@...udflare.com> wrote:
> >> On Thu, Oct 19, 2023 at 08:55 PM +08, Xuan Zhuo wrote:
> >> > On Thu, 19 Oct 2023 12:16:24 +0200, Jakub Sitnicki <jakub@...udflare.com> wrote:
> >> >> Since commit 19e226e8cc5d ("virtio: Make vp_set_vq_affinity() take a
> >> >> mask.") it is actually not needed to have a local copy of the cpu mask.
> >> >
> >> >
> >> > Could you give more info to prove this?
> >
> >
> > Actually, my question is that can we pass a val on the stack(or temp value) to
> > the irq_set_affinity_hint()?
> >
> > Such as the virtio-net uses zalloc_cpumask_var to alloc a cpu_mask, and
> > that will be released.
> >
> >
> >
> > 	int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
> > 				      bool setaffinity)
> > 	{
> > 		unsigned long flags;
> > 		struct irq_desc *desc = irq_get_desc_lock(irq, &flags, IRQ_GET_DESC_CHECK_GLOBAL);
> >
> > 		if (!desc)
> > 			return -EINVAL;
> > ->		desc->affinity_hint = m;
> > 		irq_put_desc_unlock(desc, flags);
> > 		if (m && setaffinity)
> > 			__irq_set_affinity(irq, m, false);
> > 		return 0;
> > 	}
> > 	EXPORT_SYMBOL_GPL(__irq_apply_affinity_hint);
> >
> > The above code directly refers the mask pointer. If the mask is a temp value, I
> > think that is a bug.
>
> You are completely right. irq_set_affinity_hint stores the mask pointer.
> irq_affinity_hint_proc_show later dereferences it when user reads out
> /proc/irq/*/affinity_hint.
>
> I have failed to notice that. That's why we need cpumask_copy to stay.
>
> My patch is buggy. Please disregard.
>
> I will send a v2 to only migrate from deprecated irq_set_affinity_hint.
>
> > And I notice that many places directly pass the temp value to this API.
> > And I am a little confused. ^_^ Or I missed something.
>
> There seem two be two gropus of callers:
>
> 1. Those that use get_cpu_mask/cpumask_of/cpumask_of_node to produce a
>    cpumask pointer which is a preallocated constant.
>
>    $ weggli 'irq_set_affinity_hint(_, $func(_));' ~/src/linux
>
> 2. Those that pass a pointer to memory somewhere.
>
>    $ weggli 'irq_set_affinity_hint(_, $mask);' ~/src/linux
>
> (weggli tool can be found at https://github.com/weggli-rs/weggli)
>
> I've looked over the callers from group #2 but I couldn't find any
> passing a pointer memory on stack :-)

Pls check stmmac_request_irq_multi_msi()

Thanks.


>
> Thanks for pointing this out.
>
> [...]