Date: Thu, 9 Sep 2010 14:32:52 -0400 From: John Baldwin <jhb@freebsd.org> To: Weongyo Jeong <weongyo@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: about in_multi_mtx @ netinet/in_mcast.c:1095 Message-ID: <201009091432.52066.jhb@freebsd.org> In-Reply-To: <20100909174146.GG1328@weongyo> References: <20100908201419.GF1328@weongyo> <201009090936.19412.jhb@freebsd.org> <20100909174146.GG1328@weongyo>
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On Thursday, September 09, 2010 1:41:46 pm Weongyo Jeong wrote: > On Thu, Sep 09, 2010 at 09:36:19AM -0400, John Baldwin wrote: > > 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? > > Frankly speaking I also don't know why hps@ uses sx lock. That is one > of things I'd like to change it. > > Just looking the comment at usb_request.c@441: > > /* > * Grab the default sx-lock so that serialisation > * is achieved when multiple threads are involved: > */ > sx_xlock(&udev->ctrl_sx); > > I think he might want to hold the lock even if the thread is going into > sleep. It might be for serialization. > > However even if we succeed to change the lock from sx to mutex, it's > hard to avoid the requests going into the sleep. It means USB stack > should call like below: > > mtx_sleep(chan, IN_MULTI_LOCK, ...); > > to avoid the kernel's complain (would be `sleep with holding > non-sleepable lock'). > > What I'd like to say is that the sleeping is big problem if mutex is > used that it'd be worse when multiple mutex locks are used. > > So I'm looking for a fundamental solution to solve this problem. > Welcomes any ideas. It is probably fine to just schedule a task to do the actual work of axe_setmulti(). I think you do not need to lock IN_MULTI_LOCK yourself in your task handler as long as your handler holds the appropriate lock (if_maddr_rlock() IIRC) when walking the interface's multicast address list. -- John Baldwin
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