linux-kernel - Re: [PATCH 3/3] virtio-pci: Use cpumask

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Message-Id: <3D264F7F-624D-4E9D-A139-F1DB0CC6045C@redhat.com>
Date:   Thu, 28 Apr 2022 11:48:01 +0200
From:   Christophe Marie Francois Dupont de Dinechin <cdupontd@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Christophe de Dinechin <dinechin@...hat.com>, trivial@...nel.org,
        Ben Segall <bsegall@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
Subject: Re: [PATCH 3/3] virtio-pci: Use cpumask_available to fix compilation
 error



> On 15 Apr 2022, at 10:48, Michael S. Tsirkin <mst@...hat.com> wrote:
> 
> On Thu, Apr 14, 2022 at 05:08:55PM +0200, Christophe de Dinechin wrote:
>> With GCC 12 and defconfig, we get the following error:
>> 
>> |   CC      drivers/virtio/virtio_pci_common.o
>> | drivers/virtio/virtio_pci_common.c: In function ‘vp_del_vqs’:
>> | drivers/virtio/virtio_pci_common.c:257:29: error: the comparison will
>> |  always evaluate as ‘true’ for the pointer operand in
>> |  ‘vp_dev->msix_affinity_masks + (sizetype)((long unsigned int)i * 8)’
>> |  must not be NULL [-Werror=address]
>> |   257 |                         if (vp_dev->msix_affinity_masks[i])
>> |       |                             ^~~~~~
>> 
>> This happens in the case where CONFIG_CPUMASK_OFFSTACK is not defined,
>> since we typedef cpumask_var_t as an array. The compiler is essentially
>> complaining that an array pointer cannot be NULL. This is not a very
>> important warning, but there is a function called cpumask_available that
>> seems to be defined just for that case, so the fix is easy.
>> 
>> Signed-off-by: Christophe de Dinechin <christophe@...echin.org>
>> Signed-off-by: Christophe de Dinechin <dinechin@...hat.com>
> 
> There was an alternate patch proposed for this by
> Murilo Opsfelder Araujo. What do you think about that approach?

I responded on the other thread, but let me share the response here:

[to muriloo@...ux.ibm.com]
Apologies for the delay in responding, broken laptop…

In the case where CONFIG_CPUMASK_OFFSTACK is not defined, we have:

	typedef struct cpumask cpumask_var_t[1];

So that vp_dev->msix_affinity_masks[i] is statically not null (that’s the warning)
but also a static pointer, so not kfree-safe IMO.

> 
> 
>> ---
>> drivers/virtio/virtio_pci_common.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
>> index d724f676608b..5c44a2f13c93 100644
>> --- a/drivers/virtio/virtio_pci_common.c
>> +++ b/drivers/virtio/virtio_pci_common.c
>> @@ -254,7 +254,7 @@ void vp_del_vqs(struct virtio_device *vdev)
>> 
>> 	if (vp_dev->msix_affinity_masks) {
>> 		for (i = 0; i < vp_dev->msix_vectors; i++)
>> -			if (vp_dev->msix_affinity_masks[i])
>> +			if (cpumask_available(vp_dev->msix_affinity_masks[i]))
>> 				free_cpumask_var(vp_dev->msix_affinity_masks[i]);
>> 	}
>> 
>> -- 
>> 2.35.1
>