Setting vlan flags needlessly takes the exclusive VLAN_XLOCK().
If we have stacked vlan devices (i.e. QinQ) and we set vlan flags (e.g.
IFF_PROMISC) we call rtnl_handle_ifevent() to send a notification about
the interface.
This ends up calling SIOCGIFMEDIA, which requires the VLAN_SLOCK().
Trying to take that one with the VLAN_XLOCK() held deadlocks us.
There's no need for the exclusive lock though, as we're only accessing
parent/trunk information, not modifying it, so a shared lock is
sufficient.
While here also add a test case for this issue.
Backtrace:
shared lock of (sx) vlan_sx @ /usr/src/sys/net/if_vlan.c:2192
while exclusively locked from /usr/src/sys/net/if_vlan.c:2307
panic: excl->share
cpuid = 29
time = 1683873033
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe015d4ad4b0
vpanic() at vpanic+0x152/frame 0xfffffe015d4ad500
panic() at panic+0x43/frame 0xfffffe015d4ad560
witness_checkorder() at witness_checkorder+0xcb5/frame 0xfffffe015d4ad720
_sx_slock_int() at _sx_slock_int+0x67/frame 0xfffffe015d4ad760
vlan_ioctl() at vlan_ioctl+0xf8/frame 0xfffffe015d4ad7c0
dump_iface() at dump_iface+0x12f/frame 0xfffffe015d4ad840
rtnl_handle_ifevent() at rtnl_handle_ifevent+0xab/frame 0xfffffe015d4ad8c0
if_setflag() at if_setflag+0xf6/frame 0xfffffe015d4ad930
ifpromisc() at ifpromisc+0x2a/frame 0xfffffe015d4ad960
vlan_setflags() at vlan_setflags+0x60/frame 0xfffffe015d4ad990
vlan_ioctl() at vlan_ioctl+0x216/frame 0xfffffe015d4ad9f0
if_setflag() at if_setflag+0xe4/frame 0xfffffe015d4ada60
ifpromisc() at ifpromisc+0x2a/frame 0xfffffe015d4ada90
bridge_ioctl_add() at bridge_ioctl_add+0x499/frame 0xfffffe015d4adb10
bridge_ioctl() at bridge_ioctl+0x328/frame 0xfffffe015d4adbc0
ifioctl() at ifioctl+0x972/frame 0xfffffe015d4adcc0
kern_ioctl() at kern_ioctl+0x1fe/frame 0xfffffe015d4add30
sys_ioctl() at sys_ioctl+0x154/frame 0xfffffe015d4ade00
amd64_syscall() at amd64_syscall+0x140/frame 0xfffffe015d4adf30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe015d4adf30
--- syscall (54, FreeBSD ELF64, ioctl), rip = 0x22b0f0ef8d8a, rsp = 0x22b0ec63f2c8, rbp = 0x22b0ec63f380 ---
KDB: enter: panic
[ thread pid 5715 tid 101132 ]
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