Date: Thu, 9 Sep 2010 09:36:19 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org, Weongyo Jeong <weongyo@freebsd.org> Cc: current@freebsd.org Subject: Re: about in_multi_mtx @ netinet/in_mcast.c:1095 Message-ID: <201009090936.19412.jhb@freebsd.org> In-Reply-To: <20100908201419.GF1328@weongyo> References: <20100908201419.GF1328@weongyo>
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On Wednesday, September 08, 2010 4:14:19 pm Weongyo Jeong wrote: > 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? Why is USB using an sx lock instead of a mutex? -- John Baldwin
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