Re: [PATCH] genirq/affinity: Assign default affinity to pre/post vectors

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------------------ Original ------------------
From:  "Thomas Gleixner"<tglx@xxxxxxxxxxxxx>;
Date:  Wed, Jan 16, 2019 05:26 PM
To:  "éåæ"<chenhc@xxxxxxxxxx>;
Cc:  "linux-kernel"<linux-kernel@xxxxxxxxxxxxxxx>; "Fuxin Zhang"<zhangfx@xxxxxxxxxx>; "wuzhangjin"<wuzhangjin@xxxxxxxxx>; "stable"<stable@xxxxxxxxxxxxxxx>; "Christoph Hellwig"<hch@xxxxxx>; "Michael Hernandez"<michael.hernandez@xxxxxxxxxx>;
Subject:  Re: [PATCH] genirq/affinity: Assign default affinity to pre/post vectors
 
Chen,

On Wed, 16 Jan 2019, éåæ wrote:

> please do not top-post and use line breaks around 78 char.

> I'm not removing all return NULL of irq_create_affinity_masks(), so the ......

> > Moved content to the place where it belongs so the context is preserved.

> ------------------ Original ------------------
> From:  "Thomas Gleixner"<tglx@xxxxxxxxxxxxx>;
> Date:  Wed, Jan 16, 2019 03:10 AM
> To:  "Huacai Chen"<chenhc@xxxxxxxxxx>;
> Cc:  "linux-kernel"<linux-kernel@xxxxxxxxxxxxxxx>; "Fuxin Zhang"<zhangfx@xxxxxxxxxx>; "wuzhangjin"<wuzhangjin@xxxxxxxxx>; "stable"<stable@xxxxxxxxxxxxxxx>; "Christoph Hellwig"<hch@xxxxxx>; "Michael Hernandez"<michael.hernandez@xxxxxxxxxx>;
> Subject:  Re: [PATCH] genirq/affinity: Assign default affinity to pre/post vectors

> And please configure your e-mail client to NOT copy the full headers into
> the reply.

> > > On Mon, 31 Dec 2018, Huacai Chen wrote:
> > > 
> > > > Generally, irq_create_affinity_masks() assign default affinity to pre/
> > > > post vectors correctly. However, it ignore the case that there are only
> > > > pre/post vectors (when nvecs == affd->pre_vectors + affd->post_vectors)
> > > > and return NULL. This case usually happens when nvecs = 1 (e.g. in nvme
> > > > driver when MSI-X is unavailable and fallback to MSI) and will trigger
> > > > the warning in pci_irq_get_affinity(). This patch fix the corner case.
> > > 
> > > Errm. This is just wrong. When this function returns NULL, then it has
> > > failed and the caller or any subsequent code is not supposed to use the
> > > result.
> > > 
> > > The function can return NULL for other reasons, e.g. when the memory
> > > allocation failed. How are you going to duct tape that one?
> >
> > I'm not removing all return NULL of irq_create_affinity_masks(), so the 
> > memory allocation failure still return NULL. I just handle the case that
> > there are not enough irq vectors. E.g. in nvme driver, the caller may call
> > irq_create_affinity_masks() with nvecs=1,pre_vectors=1,post_vectors=0. In
> > this case, the only one vector's default affinity assigning is skipped.
> 
> I did not say that you removed all NULL returns. I said that this function
> can return NULL for other reasons and then the same situation will happen.
> 
> If the masks pointer returned is NULL then the calling code or any
> subsequent usage needs to handle it properly. Yes, I understand that this
> change makes the warning go away for that particular case, but that's not
> making it any more correct.

Hi, Thomas,

I don't think "nvecs == affd->pre_vectors + affd->post_vectors" is an ERROR,
so it should be different with "return NULL for other reasons" to the caller. If
the caller fallback from MSI-X to MSI, it is probably "nvecs=1,pre_vectors=1,
post_vectors=0". The caller can work perfectly, if pre/post vectors are filled
with the default affinity.

> Thanks,

> tglx