Re: [PATCH] virtio_net: Fix open <-> interrupt race

[Jeff Garzik <jeff@garzik.org>]

Christian Borntraeger wrote:
> Jeff,
> 
> Rusty is (supposed to be) on vacation. Can you send this fix against
> virtio_net with your next network driver fixes for 2.6.25? 
> 
> Thank you
> 
> -
> 
> I got the following oops during interface ifup. Unfortunately its not 
> easily reproducable so I cant say for sure that my fix fixes this
> problem, but I am confident and I think its correct anyway:
> 
>    <2>kernel BUG at /space/kvm/drivers/virtio/virtio_ring.c:234!
>     <4>illegal operation: 0001 [#1] PREEMPT SMP
>     <4>Modules linked in:
>     <4>CPU: 0 Not tainted 2.6.24zlive-guest-07293-gf1ca151-dirty #91
>     <4>Process swapper (pid: 0, task: 0000000000800938, ksp: 000000000084ddb8)
>     <4>Krnl PSW : 0404300180000000 0000000000466374 (vring_disable_cb+0x30/0x34)
>     <4>           R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:0 CC:3 PM:0 EA:3
>     <4>Krnl GPRS: 0000000000000001 0000000000000001 0000000010003800 0000000000466344
>     <4>           000000000e980900 00000000008848b0 000000000084e748 0000000000000000
>     <4>           000000000087b300 0000000000001237 0000000000001237 000000000f85bdd8
>     <4>           000000000e980920 00000000001137c0 0000000000464754 000000000f85bdd8
>     <4>Krnl Code: 0000000000466368: e3b0b0700004        lg      %r11,112(%r11)
>     <4>           000000000046636e: 07fe                bcr     15,%r14
>     <4>           0000000000466370: a7f40001            brc     15,466372
>     <4>          >0000000000466374: a7f4fff6            brc     15,466360
>     <4>           0000000000466378: eb7ff0500024        stmg    %r7,%r15,80(%r15)
>     <4>           000000000046637e: a7f13e00            tmll    %r15,15872
>     <4>           0000000000466382: b90400ef            lgr     %r14,%r15
>     <4>           0000000000466386: a7840001            brc     8,466388
>     <4>Call Trace:
>     <4>([<000201500f85c000>] 0x201500f85c000)
>     <4> [<0000000000466556>] vring_interrupt+0x72/0x88
>     <4> [<00000000004801a0>] kvm_extint_handler+0x34/0x44
>     <4> [<000000000010d22c>] do_extint+0xbc/0xf8
>     <4> [<0000000000113f98>] ext_no_vtime+0x16/0x1a
>     <4> [<000000000010a182>] cpu_idle+0x216/0x238
>     <4>([<000000000010a162>] cpu_idle+0x1f6/0x238)
>     <4> [<0000000000568656>] rest_init+0xaa/0xb8
>     <4> [<000000000084ee2c>] start_kernel+0x3fc/0x490
>     <4> [<0000000000100020>] _stext+0x20/0x80
>     <4>
>     <4> <0>Kernel panic - not syncing: Fatal exception in interrupt
>     <4>
> 
> After looking at the code and the dump I think the following scenario
> happened: Ifup was running on cpu2 and the interrupt arrived on cpu0.
> Now virtnet_open on cpu 2 managed to execute napi_enable and disable_cb
> but did not execute rx_schedule. Meanwhile on cpu 0 skb_recv_done was
> called by vring_interrupt, executed netif_rx_schedule_prep, which
> succeeded and therefore called disable_cb. This triggered the BUG_ON,
> as interrupts were already disabled by cpu 2.
> 
> I think the proper solution is to make the call to disable_cb depend on
> the atomic update of NAPI_STATE_SCHED by using netif_rx_schedule_prep
> in the same way as skb_recv_done. 
> 
> 
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Acked-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> ---
>  drivers/net/virtio_net.c |   10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> Index: kvm/drivers/net/virtio_net.c
> ===================================================================
> --- kvm.orig/drivers/net/virtio_net.c
> +++ kvm/drivers/net/virtio_net.c
> @@ -321,10 +321,12 @@ static int virtnet_open(struct net_devic
>  
>  	/* If all buffers were filled by other side before we napi_enabled, we
>  	 * won't get another interrupt, so process any outstanding packets
> -	 * now.  virtnet_poll wants re-enable the queue, so we disable here. */
> -	vi->rvq->vq_ops->disable_cb(vi->rvq);
> -	netif_rx_schedule(vi->dev, &vi->napi);
> -
> +	 * now.  virtnet_poll wants re-enable the queue, so we disable here.
> +	 * We synchronize against interrupts via NAPI_STATE_SCHED */
> +	if (netif_rx_schedule_prep(dev, &vi->napi)) {
> +		vi->rvq->vq_ops->disable_cb(vi->rvq);
> +		__netif_rx_schedule(dev, &vi->napi);
> +	}
>  	return 0;
>  }

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