6.6-stable review patch. If anyone has any objections, please let me know.
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From: Jakub Kicinski kuba@kernel.org
[ Upstream commit 2da4def0f487f24bbb0cece3bb2bcdcb918a0b72 ]
Paolo spotted hangs in NIPA running driver tests against virtio. The tests hang in virtnet_close() -> virtnet_napi_tx_disable().
The problem is only reproducible if running multiple of our tests in sequence (I used TEST_PROGS="xdp.py ping.py netcons_basic.sh \ netpoll_basic.py stats.py"). Initial suspicion was that this is a simple case of double-disable of NAPI, but instrumenting the code reveals:
Deadlocked on NAPI ffff888007cd82c0 (virtnet_poll_tx): state: 0x37, disabled: false, owner: 0, listed: false, weight: 64
The NAPI was not in fact disabled, owner is 0 (rather than -1), so the NAPI "thinks" it's scheduled for CPU 0 but it's not listed (!list_empty(&n->poll_list) => false). It seems odd that normal NAPI processing would wedge itself like this.
Better suspicion is that netpoll gets enabled while NAPI is polling, and also grabs the NAPI instance. This confuses napi_complete_done():
[netpoll] [normal NAPI] napi_poll() have = netpoll_poll_lock() rcu_access_pointer(dev->npinfo) return NULL # no netpoll __napi_poll() ->poll(->weight) poll_napi() cmpxchg(->poll_owner, -1, cpu) poll_one_napi() set_bit(NAPI_STATE_NPSVC, ->state) napi_complete_done() if (NAPIF_STATE_NPSVC) return false # exit without clearing SCHED
This feels very unlikely, but perhaps virtio has some interactions with the hypervisor in the NAPI ->poll that makes the race window larger?
Best I could to to prove the theory was to add and trigger this warning in napi_poll (just before netpoll_poll_unlock()):
WARN_ONCE(!have && rcu_access_pointer(n->dev->npinfo) && napi_is_scheduled(n) && list_empty(&n->poll_list), "NAPI race with netpoll %px", n);
If this warning hits the next virtio_close() will hang.
This patch survived 30 test iterations without a hang (without it the longest clean run was around 10). Credit for triggering this goes to Breno's recent netconsole tests.
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: Paolo Abeni pabeni@redhat.com Link: https://lore.kernel.org/c5a93ed1-9abe-4880-a3bb-8d1678018b1d@redhat.com Acked-by: Jason Wang jasowang@redhat.com Reviewed-by: Xuan Zhuo xuanzhuo@linux.alibaba.com Link: https://patch.msgid.link/20250726010846.1105875-1-kuba@kernel.org Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/core/netpoll.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/net/core/netpoll.c b/net/core/netpoll.c index 1a4d2a61b060..2bdb1e84c6c8 100644 --- a/net/core/netpoll.c +++ b/net/core/netpoll.c @@ -791,6 +791,13 @@ int netpoll_setup(struct netpoll *np) if (err) goto put; rtnl_unlock(); + + /* Make sure all NAPI polls which started before dev->npinfo + * was visible have exited before we start calling NAPI poll. + * NAPI skips locking if dev->npinfo is NULL. + */ + synchronize_rcu(); + return 0;
put: