Date:	Tue, 02 Dec 2014 07:06:01 +0008
From:	Jason Wang <jasowang@...hat.com>
To:	Dexuan Cui <decui@...rosoft.com>
Cc:	KY Srinivasan <kys@...rosoft.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on transfer
 failure



On Tue, Dec 2, 2014 at 1:33 PM, Dexuan Cui <decui@...rosoft.com> wrote:
>>  -----Original Message-----
>>  From: KY Srinivasan
>>  Sent: Monday, December 1, 2014 23:55 PM
>>  To: Dexuan Cui; Jason Wang
>>  Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org; 
>> driverdev-
>>  devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
>>  vkuznets@...hat.com; Haiyang Zhang
>>  Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message on 
>> transfer
>>  failure
>>  
>>  
>>  
>>  > -----Original Message-----
>>  > From: Dexuan Cui
>>  > Sent: Monday, December 1, 2014 3:01 AM
>>  > To: Jason Wang
>>  > Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org; 
>> driverdev-
>>  > devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com; 
>> KY
>>  > Srinivasan; vkuznets@...hat.com; Haiyang Zhang
>>  > Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message 
>> on transfer
>>  > failure
>>  >
>>  > > -----Original Message-----
>>  > > From: Jason Wang [mailto:jasowang@...hat.com]
>>  > > Sent: Monday, December 1, 2014 18:18 PM
>>  > > To: Dexuan Cui
>>  > > Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
>>  > > driverdev- devel@...uxdriverproject.org; olaf@...fle.de;
>>  > > apw@...onical.com; KY Srinivasan; vkuznets@...hat.com; Haiyang
>>  Zhang
>>  > > Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete message 
>> on
>>  > > transfer failure
>>  > >
>>  > >
>>  > >
>>  > > On Mon, Dec 1, 2014 at 5:47 PM, Dexuan Cui <decui@...rosoft.com>
>>  > wrote:
>>  > > >>  -----Original Message-----
>>  > > >>  From: Jason Wang [mailto:jasowang@...hat.com]
>>  > > >>  Sent: Monday, December 1, 2014 16:23 PM
>>  > > >>  To: Dexuan Cui
>>  > > >>  Cc: gregkh@...uxfoundation.org; 
>> linux-kernel@...r.kernel.org;
>>  > > >> driverdev-
>>  > > >>  devel@...uxdriverproject.org; olaf@...fle.de; 
>> apw@...onical.com;
>>  > > >> KY  Srinivasan; vkuznets@...hat.com; Haiyang Zhang
>>  > > >>  Subject: RE: [PATCH v3] hv: hv_fcopy: drop the obsolete 
>> message on
>>  > > >> transfer  failure  On Fri, Nov 28, 2014 at 7:54 PM, Dexuan 
>> Cui
>>  > > >> <decui@...rosoft.com>
>>  > > >> wrote:
>>  > > >>  >>  -----Original Message-----
>>  > > >>  >>  From: Jason Wang [mailto:jasowang@...hat.com]  >>  Sent:
>>  > > >> Friday, November 28, 2014 18:13 PM  >>  To: Dexuan Cui  >>  
>> Cc:
>>  > > >> gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;  >>
>>  > > >> driverdev-  >>  devel@...uxdriverproject.org; olaf@...fle.de;
>>  > > >> apw@...onical.com; KY  >>  Srinivasan; vkuznets@...hat.com;
>>  Haiyang
>>  > > >> Zhang  >>  Subject: RE: [PATCH v3] hv: hv_fcopy: drop the 
>> obsolete
>>  > > >> message on  >> transfer  >>  failure  >>  On Fri, Nov 28, 
>> 2014 at
>>  > > >> 4:36 PM, Dexuan Cui <decui@...rosoft.com>  >> wrote:
>>  > > >>  >>  >>  -----Original Message-----  >>  >>  From: Jason Wang
>>  > > >> [mailto:jasowang@...hat.com]  >>  >>  Sent: Friday, November 
>> 28,
>>  > > >> 2014 14:47 PM  >>  >>  To: Dexuan Cui  >>  >>  Cc:
>>  > > >> gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;  
>> >>  >>
>>  > > >> driverdev-  >>  >>  devel@...uxdriverproject.org; 
>> olaf@...fle.de;
>>  > > >> >> apw@...onical.com; KY  >>  >>  Srinivasan; 
>> vkuznets@...hat.com;
>>  > > >> Haiyang Zhang  >>  >>  Subject: Re: [PATCH v3] hv: hv_fcopy: 
>> drop
>>  > > >> the obsolete message  >> on  >>  >> transfer  >>  >>  
>> failure  >>
>>  > > >> >>  On Thu, Nov 27, 2014 at 9:09 PM, Dexuan Cui  >>
>>  > > >> <decui@...rosoft.com>  >>  >> wrote:
>>  > > >>  >>  >>  > In the case the user-space daemon crashes, hangs 
>> or is
>>  > > >> >> killed, we  >>  >>  > need to down the semaphore, 
>> otherwise,
>>  > > >> after the daemon starts  >>  >> next  >>  >>  > time, the 
>> obsolete
>>  > > >> data in fcopy_transaction.message or  >>  >>  >
>>  > > >> fcopy_transaction.fcopy_msg will be used immediately.
>>  > > >>  >>  >>  >
>>  > > >>  >>  >>  > Cc: Jason Wang <jasowang@...hat.com>  >>  >>  > 
>> Cc:
>>  > > >> Vitaly Kuznetsov <vkuznets@...hat.com>  >>  >>  > Cc: K. Y.
>>  > > >> Srinivasan <kys@...rosoft.com>  >>  >>  > Signed-off-by: 
>> Dexuan Cui
>>  > > >> <decui@...rosoft.com>  >>  >>  > ---  >>  >>  >  >>  >>  > 
>> v2: I
>>  > > >> removed the "FCP" prefix as Greg asked.
>>  > > >>  >>  >>  >
>>  > > >>  >>  >>  >     I also updated the output message a little:
>>  > > >>  >>  >>  >     "FCP: failed to acquire the semaphore" -->
>>  > > >>  >>  >>  >     "can not acquire the semaphore: it is benign"
>>  > > >>  >>  >>  >
>>  > > >>  >>  >>  > v3: I added the code in fcopy_release() as Jason 
>> Wang
>>  > > >> >> suggested.
>>  > > >>  >>  >>  >     I removed the pr_debug (it isn't so 
>> meaningful)and
>>  > > >> added a
>>  > > >>  >>  >>  > comment instead.
>>  > > >>  >>  >>  >
>>  > > >>  >>  >>  >  drivers/hv/hv_fcopy.c | 19 +++++++++++++++++++  
>> >>  >>
>>  > > >> >  1 file changed, 19 insertions(+)  >>  >>  >  >>  >>  > 
>> diff
>>  > > >> --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c  >>  
>> >>  >
>>  > > >> index 23b2ce2..faa6ba6 100644  >>  >>  > ---
>>  > > >> a/drivers/hv/hv_fcopy.c  >>  >>  > +++ 
>> b/drivers/hv/hv_fcopy.c  >>
>>  > > >> >>  > @@ -86,6 +86,18 @@ static void fcopy_work_func(struct  
>> >>
>>  > > >> work_struct  >>  >>  > *dummy)
>>  > > >>  >>  >>  >   * process the pending transaction.
>>  > > >>  >>  >>  >   */
>>  > > >>  >>  >>  >  fcopy_respond_to_host(HV_E_FAIL);
>>  > > >>  >>  >>  > +
>>  > > >>  >>  >>  > +/* In the case the user-space daemon crashes, 
>> hangs
>>  > or
>>  > > >> is
>>  > > >>  >>  >> killed, we
>>  > > >>  >>  >>  > + * need to down the semaphore, otherwise, after
>>  > the
>>  > > >> daemon
>>  > > >>  >>  >> starts
>>  > > >>  >>  >>  > next
>>  > > >>  >>  >>  > + * time, the obsolete data in
>>  > fcopy_transaction.message
>>  > > >> or
>>  > > >>  >>  >>  > + * fcopy_transaction.fcopy_msg will be used
>>  > immediately.
>>  > > >>  >>  >>  > + *
>>  > > >>  >>  >>  > + * NOTE: fcopy_read() happens to get the
>>  > semaphore (very
>>  > > >>  >> rare)?
>>  > > >>  >>  >>  > We're
>>  > > >>  >>  >>  > + * still OK, because we've reported the failure 
>> to the
>>  > > >> host.
>>  > > >>  >>  >>  > + */
>>  > > >>  >>  >>  > +if (down_trylock(&fcopy_transaction.read_sema))
>>  > > >>  >>  >>  > +;
>>  > > >>  >>  >>
>>  > > >>  >>  >>  Sorry, I'm not quite understand how if () ; can 
>> help here.
>>  > > >>  >>  >>
>>  > > >>  >>  >>  Btw, a question not relate to this patch.
>>  > > >>  >>  >>
>>  > > >>  >>  >>  What happens if a daemon is resume from SIGSTOP and
>>  > > >> expires the  >>  >> check  >>  >>  here?
>>  > > >>  >>  > Hi Jason,
>>  > > >>  >>  > My idea is: here we need down_trylock(), but in case 
>> we
>>  > > >> can't get  >> the  >>  > semaphore, it's OK anyway:
>>  > > >>  >>  >
>>  > > >>  >>  > Scenario 1):
>>  > > >>  >>  > 1.1: when the daemon is blocked on the pread(), the 
>> daemon
>>  > > >> >> receives  >>  > SIGSTOP;  >>  > 1.2: the host user runs 
>> the
>>  > > >> PowerShell Copy-VMFile command;  >>  > 1.3.1: the driver 
>> reports
>>  > > >> the failure to the host user in 5s and  >>  > 1.3.2: the 
>> driver
>>  > > >> down()-es the semaphore;  >>  > 1.4: the daemon receives 
>> SIGCONT
>>  > > >> and it will be still blocked on  >> the  >>  > pread().
>>  > > >>  >>  > Without the down_trylock(), in 1.4, the daemon can 
>> receive
>>  > > >> an  >>  > obsolete message.
>>  > > >>  >>  > NOTE: in this scenario, the daemon is not killed.
>>  > > >>  >>  >
>>  > > >>  >>  > Scenario 2):
>>  > > >>  >>  > In senario 1), if the daemon receives SIGCONT between 
>> 1.3.1
>>  > > >> and  >> 1.3.2  >>  > and  >>  > do down() in fcopy_read(), 
>> it will
>>  > > >> receive the message but:
>>  > > >> the
>>  > > >>  >>  > driver has
>>  > > >>  >>  > reported the failure to the host user and the 
>> driver's 1.3.2
>>  > > >> can't  >>  > get the  >>  > semaphore -- IMO this is 
>> acceptably OK,
>>  > > >> though in the VM, an  >>  > incomplete  >>  > file will be 
>> left
>>  > > >> there.
>>  > > >>  >>  > BTW, I think in the daemon's hv_start_fcopy() we 
>> should add
>>  > > >> a  >>  > close(target_fd) before open()-ing a new one.
>>  > > >>  >>
>>  > > >>  >>  Right, but how about the case when resuming from 
>> SIGSTOP but
>>  > > >> no  >> timeout?
>>  > > >>  > Sorry, I don't understand this:
>>  > > >>  > if no timeout, fcopy_read() will get the semaphore and
>>  > > >> fcopy_write()
>>  > > >>  > will try to cancel fcopy_work.
>>  > > >>
>>  > > >>  Yes.
>>  > > >>  >
>>  > > >>  >
>>  > > >>  >>  Looks like in this case userspace() may wait in
>>  > > >> down_interruptible()
>>  > > >>  >>  until timeout. We probably need something like this:
>>  > > >>  >>
>>  > > >>  >>          if 
>> (down_interruptible(&fcopy_transaction.read_sema)) {
>>  > > >>  >>                  up(&fcopy_transaction.read_sema);
>>  > > >>  >>                  return -EINTR;
>>  > > >>  >>          }
>>  > > >>  > until "timeout"?
>>  > > >>  > if the daemon can't get the semaphore, it can only be 
>> wake by a
>>  > > >> > signal(the  > daemon doesn't install handler, so by 
>> default most
>>  > > >> signals will kill  > the daemon).
>>  > > >>  > In case a signal waking up the daemon doesn't kill the 
>> daemon,
>>  > > >> why  > should  > we do up()?
>>  > > >>
>>  > > >>  True, no need since we do down_trylock() in release().
>>  > > >>
>>  > > >>  Btw, there's no EINTR handling in handling pread() return 
>> value,
>>  > > >> may add such one which should be useful for something like
>>  > > >> debugging.
>>  > > >>
>>  > > >>  >
>>  > > >>  >
>>  > > >>  >>
>>  > > >>  >>  This should synchronize with the timeout work for sure.
>>  > > >>  >>  But how about only schedule it after this?
>>  > > >>  >>  It does not may sense to start the timer during 
>> interrupt  >>
>>  > > >> since the file may not even opened and it may take time  >>  
>> to
>>  > > >> handle signals?
>>  > > >>  >>
>>  > > >>  >>  >
>>  > > >>  >>  >>  >
>>  > > >>  >>  >>  > +
>>  > > >>  >>  >>  >  }
>>  > > >>  >>  >>  >
>>  > > >>  >>  >>  >  static int fcopy_handle_handshake(u32 version)  
>> >>  >>
>>  > > >> > @@ -351,6 +363,13 @@ static int fcopy_release(struct inode 
>>  >>
>>  > > >> *inode,  >>  >>  > struct file *f)
>>  > > >>  >>  >>  >   */
>>  > > >>  >>  >>  >  in_hand_shake = true;
>>  > > >>  >>  >>  >  opened = false;
>>  > > >>  >>  >>  > +
>>  > > >>  >>  >>  > +if (cancel_delayed_work_sync(&fcopy_work)) {
>>  > > >>  >>  >>  > +/* We haven't up()-ed the semaphore(very
>>  > rare)? */
>>  > > >>  >>  >>  > +if
>>  > (down_trylock(&fcopy_transaction.read_sema))
>>  > > >>  >>  >>  > +;
>>  > > >>  >>  >>
>>  > > >>  >>  >>  And this.
>>  > > >>  >>  >
>>  > > >>  >>  > Scenario 3):
>>  > > >>  >>  > When the daemon exits(e.g., SIGKILL received), if 
>> there is a
>>  > > >> >>  > fcopy_work  >>  > pending (scheduled but not start to 
>> run
>>  > > >> yet), we should cancel the  >>  > work (as you suggested) and
>>  > > >> down() the semaphore, otherwise, the  >>  > obsolete message 
>> will
>>  > > >> be received by the next instance of the  >> daemon.
>>  > > >>  >>
>>  > > >>  >>  Yes
>>  > > >>  >>  >
>>  > > >>  >>  >
>>  > > >>  >>  > Scenario 4):  in the driver's 
>> hv_fcopy_onchannelcallback():
>>  > > >>  >>  >         schedule_delayed_work(&fcopy_work, 5*HZ);
>>  > > >>  >>  >         ----> if fcopy_release() is running on 
>> another vcpu,
>>  > > >> just
>>  > > >>  >>  > before the next line?
>>  > > >>  >>  >         fcopy_send_data();
>>  > > >>  >>  >
>>  > > >>  >>  > In this case, fcopy_release() can cancel fcopy_work, 
>> but  >>
>>  > > >> > can't get the semaphore since it hasn't been up()-ed.
>>  > > >>  >>  > Hmm, in this case,   fcopy_send_data() will do up()  
>> later,
>>  > > >> and
>>  > > >>  >> we'll
>>  > > >>  >>  > buffer an obsolete message in the driver, and the 
>> message
>>  > > >> will be  >>  > fetched by the next instance of the daemon...
>>  > > >>  >>  >
>>  > > >>  >>  > Looks we need a spinlock here?
>>  > > >>  >>
>>  > > >>  >>  Unless fcopy_release() can wait for all data for current
>>  > > >> transation  >>  to be received. Spinlock won't help.
>>  > > >>  >>
>>  > > >>  >>  But an idea is let the daemon the handle such cases. 
>> E.g make
>>  > > >> sure  >> the  >>  processing begins with START_COPY and end 
>> with
>>  > > >> COMPLETE/CANCEL_COPY.
>>  > > >>  >>  Drop all requests that does not start with START_COPY.
>>  > > >>  >>
>>  > > >>  >>  Thought?
>>  > > >>  > Good idea.
>>  > > >>  > I also think we should reinforce the concept of state 
>> machine in
>>  > > >> the  > daemon code.
>>  > > >>
>>  > > >>  Yes, it needs.
>>  > > > I agree.
>>  > > > Obviously we can do something to make the daemon/driver work 
>> better
>>  > > > in the corner cases.
>>  > > >
>>  > > >>  >
>>  > > >>  > The daemon/driver communication has so many corner 
>> cases...
>>  > > >>
>>  > > >>  Looks so, let's first address the issue mentioned in this 
>> patch.
>>  > > > OK.
>>  > > >
>>  > > >>  I don't have any more comments other than changing
>>  > > >>
>>  > > >>   if(down_trylock(&fcopy_transaction.read_sema))
>>  > > >>       ;
>>  > > >>
>>  > > >>  to
>>  > > >>
>>  > > >>  down_trylock(&fcopy_transaction.read_sema);
>>  > > > Hi Jason,
>>  > > > This is to address Vitaly's comment in the bugzilla:
>>  > > > https://bugzilla.redhat.com/show_bug.cgi?id=1162100#c5
>>  > > >
>>  > > > down_trylock(&fcopy_transaction.read_sema) will
>>  > > >
>>  > > > "
>>  > > > produces the following compile warning:
>>  > > > drivers/hv/hv_fcopy.c: In function ‘fcopy_work_func’:
>>  > > > drivers/hv/hv_fcopy.c:95:2: warning: ignoring return value of
>>  > > > ‘down_trylock’, declared with attribute warn_unused_result
>>  > > > [-Wunused-result]
>>  > > >   (void)down_trylock(&fcopy_transaction.read_sema);
>>  > > > "
>>  > > >
>>  > > > Actually I personally don't care about the warning, because 
>> we only
>>  > > > see it when we run some kind of code checker program.  :-)
>>  > > >
>>  > > > I can change my v3  to the "normal" style you prefer, if 
>> there is no
>>  > > > strong objection from Vitaly?
>>  > >
>>  > > Ah, I see the point. Then no objection for this patch.
>>  > >
>>  > > Since Vitaly said he does not has objection.
>>  > >
>>  > > Acked-by: Jason Wang <jasowang@...hat.com>
>>  >
>>  > I realized non-trivial effort might be needed to better handle 
>> corner cases.
>>  > This is only the first step.
>>  >
>>  > E.g., I found a new corner case:
>>  > after we unload the hv_utils/fcopy driver, the fcopy daemon can 
>> exit and
>>  > fcopy_release() will be invoked -- at this time fcopy_release() 
>> has become
>>  > obsolete and invalid, and as a result, a kernel panic can happen: 
>>  I did see
>>  the
>>  > panic once.
>>  
>>  You should not be able to unload the driver when there is still an 
>> open
>>  reference from the
>>  Daemon.
>>  
>>  K. Y
> 
> Hi KY, I'm able to reproduce the below issue every time:
> (I ran the below in Ubuntu 14.10 but I think we're able to repro it 
> in any distro):
> 
> 1) start hv_fcopy_daemon and use ps to make sure it's running in the 
> background;
> 2) kill -STOP `pidof hv_fcopy_daemon`, and use ps to make sure the 
> daemon is in
> the state of "Ts";
> 3) rmmod hv_utils  # dexuan: this can runs fine. 

Probably the opened file should hold a refcnt of the module.
> 
> 4) kill -CONT `pidof hv_fcopy_daemon` and we see an oops at once:
> 
> [ 9699.421564] BUG: unable to handle kernel paging request at 
> ffffffffc019c230
> [ 9699.422701] IP: [<ffffffff811de294>] filp_close+0x24/0x70
> [ 9699.423154] PGD 1c16067 PUD 1c18067 PMD 366d8067 PTE 0
> [ 9699.423154] Oops: 0000 [#1] SMP
> [ 9699.423154] Modules linked in: hv_utils rfcomm bnep bluetooth 
> 6lowpan_iphc joydev hid_generic crct10dif_pclmul crc32_pclmul 
> ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper 
> ablk_helper cryptd hid_hyperv serio_raw hid hyperv_keyboard 
> hv_balloon mac_hid hyperv_fb i2c_piix4 parport_pc ppdev lp parport 
> hv_storvsc psmouse floppy hv_vmbus tulip pata_acpi [last unloaded: 
> hv_utils]
> [ 9699.423154] CPU: 0 PID: 4659 Comm: hv_fcopy_daemon Not tainted 
> 3.16.0-23-generic #31-Ubuntu
> [ 9699.423154] Hardware name: Microsoft Corporation Virtual 
> Machine/Virtual Machine, BIOS 090006  05/23/2012
> [ 9699.423154] task: ffff880019ce5bb0 ti: ffff8800c7be0000 task.ti: 
> ffff8800c7be0000
> [ 9699.423154] RIP: 0010:[<ffffffff811de294>]  [<ffffffff811de294>] 
> filp_close+0x24/0x70
> [ 9699.423154] RSP: 0018:ffff8800c7be3e48  EFLAGS: 00010246
> [ 9699.423154] RAX: ffffffffc019c1c0 RBX: ffff8800c7b9c200 RCX: 
> ffff8800396548d8
> [ 9699.423154] RDX: 0000000080000000 RSI: ffff880039654880 RDI: 
> ffff8800c7b9c200
> [ 9699.423154] RBP: ffff8800c7be3e60 R08: 0000000000000001 R09: 
> 00000001802a0017
> [ 9699.423154] R10: ffff88003de17640 R11: ffff880087963140 R12: 
> 0000000000000000
> [ 9699.423154] R13: ffff880039654880 R14: ffff880039654890 R15: 
> 0000000000000004
> [ 9699.423154] FS:  0000000000000000(0000) GS:ffff88003de00000(0000) 
> knlGS:0000000000000000
> [ 9699.423154] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 9699.423154] CR2: ffffffffc019c230 CR3: 0000000001c13000 CR4: 
> 00000000001406f0
> [ 9699.423154] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 
> 0000000000000000
> [ 9699.423154] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 
> 0000000000000400
> [ 9699.423154] Stack:
> [ 9699.423154]  0000000000000001 0000000000000000 ffff880039654880 
> ffff8800c7be3ea0
> [ 9699.423154]  ffffffff811ff208 00000001034a4c80 ffff880019ce5bb0 
> ffff880039654880
> [ 9699.423154]  ffff880019ce6308 ffff8800126b8e00 ffff8800126b8e60 
> ffff8800c7be3ec8
> [ 9699.423154] Call Trace:
> [ 9699.423154]  [<ffffffff811ff208>] put_files_struct+0x78/0xd0
> [ 9699.423154]  [<ffffffff811ff307>] exit_files+0x47/0x50
> [ 9699.423154]  [<ffffffff810725a7>] do_exit+0x357/0xab0
> [ 9699.423154]  [<ffffffff81072d83>] do_group_exit+0x43/0xc0
> [ 9699.423154]  [<ffffffff81072e14>] SyS_exit_group+0x14/0x20
> [ 9699.423154]  [<ffffffff81787ced>] system_call_fastpath+0x1a/0x1f
> [ 9699.423154] Code: ff ff 0f 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 
> 41 55 41 54 53 48 8b 47 38 48 89 fb 48 85 c0 74 46 48 8b 47 28 45 31 
> e4 49 89 f5 <48> 8b 40 70 48 85 c0 74 05 ff d0 41 89 c4 f6 43 45 40 
> 75 16 4c
> [ 9699.423154] RIP  [<ffffffff811de294>] filp_close+0x24/0x70
> [ 9699.423154]  RSP <ffff8800c7be3e48>
> [ 9699.423154] CR2: ffffffffc019c230
> [ 9699.423154] ---[ end trace 0278723740833669 ]---
> [ 9699.423154] Fixing recursive fault but reboot is needed!
> 
> I understand an administrator should not unload the driver before 
> stopping the
> daemon, but this could happen by accident.
> 
>  -- Dexuan
> 
> �NrybXǧv^)޺{.n+{zX.ܨ}Ơz&j:+v.zZ++zfh~iz.w?&)ߢ.f^jǫym@Aa.0h.i

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