LOR in /usr/src/sys/netinet/in.c:972

Robert Watson rwatson at FreeBSD.org
Thu Sep 15 12:24:18 PDT 2005

On Thu, 15 Sep 2005, John Baldwin wrote:

> On Saturday 03 September 2005 10:28 am, Dan Cojocar wrote:
>> Hello,
>> Here is my second lor for today :) that i didn't find on
>> http://sources.zabbadoz.net/freebsd/lor.html
>> I get this when i run dhclient on cdce0 interface. It's harmless, because I
>> obtain an valid ip and everything is working ok.
>> lock order reversal
>> 1st 0xc06a2e40 in_multi_mtx (in_multi_mtx) @ /usr/src/sys/netinet/in.c:972
>> 2nd 0xc0655660 Giant (Giant) @ /usr/src/sys/net/if.c:1998
>> KDB: stack backtrace:
>> witness_checkorder(c0655660,9,c0612eef,7ce,12c) at witness_checkorder+0x51c
>> _mtx_lock_flags(c0655660,0,c0612eef,7ce,c) at _mtx_lock_flags+0x54
>> if_addmulti(c1a67800,d5729afc,d5729af8,3cc,c1bf0780) at if_addmulti+0x260
>> in_addmulti(d5729b40,c1a67800,1,2cc,c1b8bbc8) at in_addmulti+0x66
>> in_ifinit(c1f53190,0,0,0,d5729b90) at in_ifinit+0x5bc
>> in_control(c1f8b000,8040691a,c1f53180,c1a67800,c1aa0e10) at
>> in_control+0xfaa ifioctl(c1f8b000,8040691a,c1f53180,c1aa0e10,2) at
>> ifioctl+0x15a
>> soo_ioctl(c1f5e000,8040691a,c1f53180,c1f65200,c1aa0e10) at soo_ioctl+0x2e8
>> ioctl(c1aa0e10,d5729d04,c,422,3) at ioctl+0x118
>> syscall(3b,3b,3b,808e3a0,0) at syscall+0x13d
>> Xint0x80_syscall() at Xint0x80_syscall+0x1f
>> --- syscall (54, FreeBSD ELF32, ioctl), eip = 0x807f49b, esp = 0xbfbfe5bc,
>> ebp = 0xbfbfee28 ---
> This is a real LOR caused by the fact that in_addmulti() holds the 
> in_multi_mtx() lock across the call to if_addmulti() and if_addmulti() 
> will acquire Giant for non-MPsafe drivers around calls into their ioctl 
> routine. I think that to better expose these issues, the various 
> conditional-Giant macros need to include a witness_warn() to make sure 
> only sleepable locks and/or Giant are held when the lock macro is 
> invoked.

The right strategy here may in fact be to eliminate Giant acquisition by 
network device drivers...

Robert N M Watson

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