Date: Wed, 8 Sep 2010 13:14:19 -0700 From: Weongyo Jeong <weongyo.jeong@gmail.com> To: current@freebsd.org Subject: about in_multi_mtx @ netinet/in_mcast.c:1095 Message-ID: <20100908201419.GF1328@weongyo>
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Hello, I have a question about IN_MULTI_LOCK() because it uses MTX_DEF flag when it's initialized so I always encounters the following LOR lock order reversal: (sleepable after non-sleepable) 1st 0xffffffff80d0b560 in_multi_mtx (in_multi_mtx) @ netinet/in_mcast.c:1095 2nd 0xffffff00014e3850 USB device SX lock (USB device SX lock) @ /usr/home/freebsd_usb/sys/modules/usb/usb/../../../dev/usb/usb_request.c:441 KDB: stack backtrace: db_trace_self_wrapper() at db_trace_self_wrapper+0x2a _witness_debugger() at _witness_debugger+0x2e witness_checkorder() at witness_checkorder+0x807 _sx_xlock() at _sx_xlock+0x55 usbd_do_request_flags() at usbd_do_request_flags+0xe5 axe_cmd() at axe_cmd+0xc7 axe_setmulti_locked() at axe_setmulti_locked+0x70 axe_setmulti() at axe_setmulti+0x3e axe_ioctl() at axe_ioctl+0x132 if_addmulti() at if_addmulti+0x19b in_joingroup_locked() at in_joingroup_locked+0x1bc in_joingroup() at in_joingroup+0x52 in_control() at in_control+0x1144 ifioctl() at ifioctl+0x1118 kern_ioctl() at kern_ioctl+0xbe ioctl() at ioctl+0xfd syscallenter() at syscallenter+0x1aa syscall() at syscall+0x4c Xfast_syscall() at Xfast_syscall+0xe2 when I uses the following code at driver's ioctl routine: case SIOCADDMULTI: case SIOCDELMULTI: axe_setmulti(sc, 0); break; It means that USB driver always should defer SIOCADDMULTI / SIOCDELMULTI handling to the other process context to avoid LOR. My question is that is it safe if the multicasting operations for USB device happens without IN_MULTI_LOCK? Or is there any race cases if the task is deferred? Or is it OK like the following code? case SIOCADDMULTI: case SIOCDELMULTI: IN_MULTI_UNLOCK(); axe_setmulti(sc, 0); IN_MULTI_LOCK(); break; regards, Weongyo Jeong
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