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netpoll: Fix race condition in netpoll_owner_active

KCSAN detected a race condition in netpoll:

BUG: KCSAN: data-race in net_rx_action / netpoll_send_skb
write (marked) to 0xffff8881164168b0 of 4 bytes by interrupt on cpu 10:
net_rx_action (./include/linux/netpoll.h:90 net/core/dev.c:6712 net/core/dev.c:6822)
<snip>
read to 0xffff8881164168b0 of 4 bytes by task 1 on cpu 2:
netpoll_send_skb (net/core/netpoll.c:319 net/core/netpoll.c:345 net/core/netpoll.c:393)
netpoll_send_udp (net/core/netpoll.c:?)
<snip>
value changed: 0x0000000a -> 0xffffffff

This happens because netpoll_owner_active() needs to check if the
current CPU is the owner of the lock, touching napi->poll_owner
non atomically. The ->poll_owner field contains the current CPU holding
the lock.

Use an atomic read to check if the poll owner is the current CPU.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240429100437.3487432-1-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

authored by

Breno Leitao and committed by
Jakub Kicinski c2e6a872 3b5933e9

+1 -1
+1 -1
net/core/netpoll.c
··· 316 316 struct napi_struct *napi; 317 317 318 318 list_for_each_entry_rcu(napi, &dev->napi_list, dev_list) { 319 - if (napi->poll_owner == smp_processor_id()) 319 + if (READ_ONCE(napi->poll_owner) == smp_processor_id()) 320 320 return 1; 321 321 } 322 322 return 0;