On 12/3/25 13:37, Jason Wang wrote:
On Tue, Dec 2, 2025 at 11:29 PM Bui Quang Minh <[email protected]> wrote:
On 12/2/25 13:03, Jason Wang wrote:
On Mon, Dec 1, 2025 at 11:04 PM Bui Quang Minh <[email protected]> wrote:
On 11/28/25 09:20, Jason Wang wrote:
On Fri, Nov 28, 2025 at 1:47 AM Bui Quang Minh <[email protected]> wrote:
I think the the requeue in refill_work is not the problem here. In
virtnet_rx_pause[_all](), we use cancel_work_sync() which is safe to
use "even if the work re-queues itself". AFAICS, cancel_work_sync()
will disable work -> flush work -> enable again. So if the work requeue
itself in flush work, the requeue will fail because the work is already
disabled.
Right.
I think what triggers the deadlock here is a bug in
virtnet_rx_resume_all(). virtnet_rx_resume_all() calls to
__virtnet_rx_resume() which calls napi_enable() and may schedule
refill. It schedules the refill work right after napi_enable the first
receive queue. The correct way must be napi_enable all receive queues
before scheduling refill work.
So what you meant is that the napi_disable() is called for a queue
whose NAPI has been disabled?
cpu0] enable_delayed_refill()
cpu0] napi_enable(queue0)
cpu0] schedule_delayed_work(&vi->refill)
cpu1] napi_disable(queue0)
cpu1] napi_enable(queue0)
cpu1] napi_disable(queue1)
In this case cpu1 waits forever while holding the netdev lock. This
looks like a bug since the netdev_lock 413f0271f3966 ("net: protect
NAPI enablement with netdev_lock()")?
Yes, I've tried to fix it in 4bc12818b363 ("virtio-net: disable delayed
refill when pausing rx"), but it has flaws.
I wonder if a simplified version is just restoring the behaviour
before 413f0271f3966 by using napi_enable_locked() but maybe I miss
something.
As far as I understand, before 413f0271f3966 ("net: protect NAPI
enablement with netdev_lock()"), the napi is protected by the
I guess you meant napi enable/disable actually.
rtnl_lock(). But in the refill_work, we don't acquire the rtnl_lock(),
Any reason we need to hold rtnl_lock() there?
Correct me if I'm wrong here. Before 413f0271f3966 ("net: protect NAPI
enablement with netdev_lock()"), napi_disable and napi_enable are not
safe to be called concurrently.
The example race is
napi_disable -> napi_save_config -> write to n->config->defer_hard_irqs
napi_enable -> napi_restore_config -> read n->config->defer_hard_irqs
In refill_work, we don't hold any locks so the race scenario can happen.
Maybe I misunderstand what you mean by restoring the behavior before
413f0271f3966. Do you mean that we use this pattern
In virtnet_xdp_se;
netdev_lock(dev);
virtnet_rx_pause_all()
-> napi_disable_locked
virtnet_rx_resume_all()
-> napi_disable_locked
netdev_unlock(dev);
And in other places where we pause the rx too. It will hold the
netdev_lock during the time napi is disabled so that even when
refill_work happens concurrently, napi_disable cannot acquire the
netdev_lock and gets stuck inside.
so it seems like we will have race condition before 413f0271f3966 ("net:
protect NAPI enablement with netdev_lock()").
Thanks,
Quang Minh.
Thanks
Thanks,
Quang Minh.
