From 4265f161b6bb7b31163671329b1142b9023bf4e3 Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Fri, 14 Mar 2008 14:17:05 +0100 Subject: [PATCH] virtio: fix race in enable_cb There is a race in virtio_net, dealing with disabling/enabling the callback. I saw the following oops: kernel BUG at /space/kvm/drivers/virtio/virtio_ring.c:218! illegal operation: 0001 [#1] SMP Modules linked in: sunrpc dm_mod CPU: 2 Not tainted 2.6.25-rc1zlive-host-10623-gd358142-dirty #99 Process swapper (pid: 0, task: 000000000f85a610, ksp: 000000000f873c60) Krnl PSW : 0404300180000000 00000000002b81a6 (vring_disable_cb+0x16/0x20) R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:0 CC:3 PM:0 EA:3 Krnl GPRS: 0000000000000001 0000000000000001 0000000010005800 0000000000000001 000000000f3a0900 000000000f85a610 0000000000000000 0000000000000000 0000000000000000 000000000f870000 0000000000000000 0000000000001237 000000000f3a0920 000000000010ff74 00000000002846f6 000000000fa0bcd8 Krnl Code: 00000000002b819a: a7110001 tmll %r1,1 00000000002b819e: a7840004 brc 8,2b81a6 00000000002b81a2: a7f40001 brc 15,2b81a4 >00000000002b81a6: a51b0001 oill %r1,1 00000000002b81aa: 40102000 sth %r1,0(%r2) 00000000002b81ae: 07fe bcr 15,%r14 00000000002b81b0: eb7ff0380024 stmg %r7,%r15,56(%r15) 00000000002b81b6: a7f13e00 tmll %r15,15872 Call Trace: ([<000000000fa0bcd0>] 0xfa0bcd0) [<00000000002b8350>] vring_interrupt+0x5c/0x6c [<000000000010ab08>] do_extint+0xb8/0xf0 [<0000000000110716>] ext_no_vtime+0x16/0x1a [<0000000000107e72>] cpu_idle+0x1c2/0x1e0 The problem can be triggered with a high amount of host->guest traffic. I think its the following race: poll says netif_rx_complete poll calls enable_cb enable_cb opens the interrupt mask a new packet comes, an interrupt is triggered----\ enable_cb sees that there is more work | enable_cb disables the interrupt | . V . interrupt is delivered . skb_recv_done does atomic napi test, ok some waiting disable_cb is called->check fails->bang! . poll would do napi check poll would do disable_cb The fix is to let enable_cb not disable the interrupt again, but expect the caller to do the cleanup if it returns false. In that case, the interrupt is only disabled, if the napi test_set_bit was successful. Signed-off-by: Christian Borntraeger Signed-off-by: Rusty Russell (cleaned up doco) --- drivers/net/virtio_net.c | 10 +++++++--- drivers/virtio/virtio_ring.c | 1 - include/linux/virtio.h | 5 +++-- 3 files changed, 10 insertions(+), 6 deletions(-) diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index e575df83e5c2..b58472cf76f8 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -203,8 +203,11 @@ again: if (received < budget) { netif_rx_complete(vi->dev, napi); if (unlikely(!vi->rvq->vq_ops->enable_cb(vi->rvq)) - && netif_rx_reschedule(vi->dev, napi)) + && napi_schedule_prep(napi)) { + vi->rvq->vq_ops->disable_cb(vi->rvq); + __netif_rx_schedule(vi->dev, napi); goto again; + } } return received; @@ -278,10 +281,11 @@ again: pr_debug("%s: virtio not prepared to send\n", dev->name); netif_stop_queue(dev); - /* Activate callback for using skbs: if this fails it + /* Activate callback for using skbs: if this returns false it * means some were used in the meantime. */ if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) { - printk("Unlikely: restart svq failed\n"); + printk("Unlikely: restart svq race\n"); + vi->svq->vq_ops->disable_cb(vi->svq); netif_start_queue(dev); goto again; } diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 3a28c1382131..aa714028641e 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -232,7 +232,6 @@ static bool vring_enable_cb(struct virtqueue *_vq) vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT; mb(); if (unlikely(more_used(vq))) { - vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT; END_USE(vq); return false; } diff --git a/include/linux/virtio.h b/include/linux/virtio.h index 260d1fcf29a4..12c18ac1b973 100644 --- a/include/linux/virtio.h +++ b/include/linux/virtio.h @@ -43,8 +43,9 @@ struct virtqueue * vq: the struct virtqueue we're talking about. * @enable_cb: restart callbacks after disable_cb. * vq: the struct virtqueue we're talking about. - * This returns "false" (and doesn't re-enable) if there are pending - * buffers in the queue, to avoid a race. + * This re-enables callbacks; it returns "false" if there are pending + * buffers in the queue, to detect a possible race between the driver + * checking for more work, and enabling callbacks. * * Locking rules are straightforward: the driver is responsible for * locking. No two operations may be invoked simultaneously. -- 2.20.1