From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eugene Shatokhin Subject: Re: [PATCH] usbnet: Fix two races between usbnet_stop() and the BH Date: Wed, 19 Aug 2015 14:59:01 +0300 Message-ID: <55D46F85.50608@rosalab.ru> References: <55CE1D7E.2070400@rosalab.ru> <1439571516-11862-1-git-send-email-eugene.shatokhin@rosalab.ru> <20150818.185407.1667358232705414236.davem@davemloft.net> <55D436D5.6010105@rosalab.ru> <87k2sreefu.fsf@nemi.mork.no> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: David Miller , oneukum@suse.com, netdev@vger.kernel.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org To: =?UTF-8?Q?Bj=c3=b8rn_Mork?= Return-path: In-Reply-To: <87k2sreefu.fsf@nemi.mork.no> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 19.08.2015 13:54, Bj=C3=B8rn Mork =D0=BF=D0=B8=D1=88=D0=B5=D1=82: > Eugene Shatokhin writes: > >> 19.08.2015 04:54, David Miller =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>> From: Eugene Shatokhin >>> Date: Fri, 14 Aug 2015 19:58:36 +0300 >>> >>>> 2. The second race is on dev->flags. >>>> >>>> dev->flags is set to 0 here: >>>> *0 usbnet_stop (usbnet.c:816) >>>> /* deferred work (task, timer, softirq) must also stop. >>>> * can't flush_scheduled_work() until we drop rtnl (later), >>>> * else workers could deadlock; so make workers a NOP. >>>> */ >>>> dev->flags =3D 0; >>>> del_timer_sync (&dev->delay); >>>> tasklet_kill (&dev->bh); >>>> >>>> And here, the code clears EVENT_RX_KILL bit in dev->flags, which m= ay >>>> execute concurrently with the above operation: >>>> *0 clear_bit (bitops.h:113, inlined) >>>> *1 usbnet_bh (usbnet.c:1475) >>>> /* restart RX again after disabling due to high error rate *= / >>>> clear_bit(EVENT_RX_KILL, &dev->flags); >>>> >>>> It seems, setting dev->flags to 0 is not necessarily atomic w.r.t. >>>> clear_bit() and other bit operations with dev->flags. It is safer = to >>>> make it atomic and this way, make the race harmless. >>>> >>>> While at it, the checking of EVENT_NO_RUNTIME_PM bit of dev->flags= in >>>> usbnet_stop() was fixed too: the bit should be checked before dev-= >flags >>>> is cleared. >>> >>> The fix for this is excessive. >>> >>> Instead of all of this madness, looping over expensive clear_bit() >>> atomics, just do whatever it takes to make sure that usbnet_bh() is >>> quiesced and cannot execute any more. Then you can safely clear >>> dev->flags normally. >>> >> >> If I understand it correctly, it is to make sure usbnet_bh() is not >> scheduled again that dev->flags should be set to 0 first, one way or >> another. That is what this madness is for. > > Assuming there is a race which may reorder these, exactly what > difference does it make wrt EVENT_RX_KILL if you do > > a) clear_bit(EVENT_RX_KILL, &dev->flags); > dev->flags =3D 0; > > or > > b) dev->flags =3D 0; > clear_bit(EVENT_RX_KILL, &dev->flags); > > > AFAICS, the result will be a cleared EVENT_RX_KILL bit in either case= =2E > Thanks for the review! The problem is not in the reordering but rather in the fact that=20 "dev->flags =3D 0" is not necessarily atomic w.r.t.=20 "clear_bit(EVENT_RX_KILL, &dev->flags)", and vice versa. So the following might be possible, although unlikely: CPU0 CPU1 clear_bit: read dev->flags clear_bit: clear EVENT_RX_KILL in the read value dev->flags=3D0; clear_bit: write updated dev->flags As a result, dev->flags may become non-zero again. I cannot prove yet that this is an impossible situation. If anyone can,= =20 please explain. If so, this part of the patch will not be needed. > > The EVENT_NO_RUNTIME_PM bug should definitely be fixed. Please split > that out as a separate fix. It's a separate issue, and should be > backported to all maintained stable releases it applies to (anything > from v3.8 and newer) Yes, that makes sense. However, this fix was originally provided by=20 Oliver Neukum rather than me, so I would like to hear his opinion as=20 well first. > > > Bj=C3=B8rn > Regards, Eugene