[Bug 274795] broken locking in e6000sw

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274795

            Bug ID: 274795
           Summary: broken locking in e6000sw
           Product: Base System
           Version: 15.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: markj@FreeBSD.org
                CC: mw@FreeBSD.org

Commit 469290648005e13b819a19353032ca53dda4378f made e6000sw's implementation
of miibus_(read|write)reg assume that the softc lock is held.  I presume that
is to avoid lock recursion in e6000sw_attach() -> e6000sw_attach_miibus() ->
mii_attach() -> MIIBUS_READREG().

However, the lock assertion in e6000sw_readphy_locked() can fail:

panic: Lock e6000sw not exclusively locked @
/usr/home/markj/src/freebsd/sys/dev/etherswitch/e6000sw/e6000sw.c:773

cpuid = 0
time = 1698599456
KDB: stack backtrace:
db_trace_self() at db_trace_self
db_trace_self_wrapper() at db_trace_self_wrapper+0x38
vpanic() at vpanic+0x1a0
panic() at panic+0x48
_sx_assert() at _sx_assert+0x100
e6000sw_readphy_locked() at e6000sw_readphy_locked+0x40
gentbi_probe() at gentbi_probe+0x7c
device_probe_child() at device_probe_child+0x150                               
device_probe() at device_probe+0xa0                                            
device_probe_and_attach() at device_probe_and_attach+0x38                      
bus_generic_attach() at bus_generic_attach+0x1c                                 
miibus_attach() at miibus_attach+0x88                                        
device_attach() at device_attach+0x3fc
device_probe_and_attach() at device_probe_and_attach+0x80                      
bus_generic_driver_added() at bus_generic_driver_added+0x90
devclass_driver_added() at devclass_driver_added+0x48
devclass_add_driver() at devclass_add_driver+0x148
module_register_init() at module_register_init+0xb4                             
linker_load_module() at linker_load_module+0xacc                               
kern_kldload() at kern_kldload+0x190                                           
sys_kldload() at sys_kldload+0x64                                              
do_el0_sync() at do_el0_sync+0x59c                                             
handle_el0_sync() at handle_el0_sync+0x48

In particular, gentbi_probe() obviously didn't acquire the softc lock.

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