Re: [PATCH 3/3] virtio-pci: Use cpumask_available to fix compilation error

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Christophe Marie Francois Dupont de Dinechin <cdupontd@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	trivial@kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Zhen Lei <thunder.leizhen@huawei.com>,
	linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	virtualization@lists.linux-foundation.org,
	Ben Segall <bsegall@google.com>, Ingo Molnar <mingo@redhat.com>,
	Mel Gorman <mgorman@suse.de>,
	Murilo Opsfelder Araujo <muriloo@linux.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Christophe de Dinechin <dinechin@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH 3/3] virtio-pci: Use cpumask_available to fix compilation error
Date: Thu, 28 Apr 2022 07:06:35 -0400	[thread overview]
Message-ID: <20220428070620-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <3D264F7F-624D-4E9D-A139-F1DB0CC6045C@redhat.com>

On Thu, Apr 28, 2022 at 11:48:01AM +0200, Christophe Marie Francois Dupont de Dinechin wrote:
> 
> 
> > On 15 Apr 2022, at 10:48, Michael S. Tsirkin <mst@redhat.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@dinechin.org>
> >> Signed-off-by: Christophe de Dinechin <dinechin@redhat.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@linux.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.

Not sure I understand what you are saying here.

> > 
> > 
> >> ---
> >> 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
> > 

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