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authorChristian Borntraeger <borntraeger@de.ibm.com>2008-03-14 14:17:05 +0100
committerRusty Russell <rusty@rustcorp.com.au>2008-03-17 22:58:21 +1100
commit4265f161b6bb7b31163671329b1142b9023bf4e3 (patch)
tree1494768aaf38407ca8f1336f6ae80301b27135bb
parentda74e89d40995600b3b07ac500084920247687ce (diff)
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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 <borntraeger@de.ibm.com> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (cleaned up doco)
-rw-r--r--drivers/net/virtio_net.c10
-rw-r--r--drivers/virtio/virtio_ring.c1
-rw-r--r--include/linux/virtio.h5
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.