Date:	Wed, 28 Aug 2013 09:50:43 +0800
From:	vaughan <vaughan.cao@...cle.com>
To:	dgilbert@...erlog.com
CC:	Jörn Engel <joern@...fs.org>,
	JBottomley@...allels.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/4] [SCSI] sg: fix race condition in sg_open

On 08/27/2013 09:13 PM, Douglas Gilbert wrote:
> On 13-08-27 10:16 AM, vaughan wrote:
>> On 08/13/2013 11:16 AM, Douglas Gilbert wrote:
>>> On 13-08-12 10:46 PM, vaughan wrote:
>>>> On 08/06/2013 04:52 AM, Douglas Gilbert wrote:
>>>>> On 13-08-04 10:19 PM, vaughan wrote:
>>>>>> On 08/03/2013 01:25 PM, Douglas Gilbert wrote:
>>>>>>> On 13-08-01 01:01 AM, Douglas Gilbert wrote:
>>>>>>>> On 13-07-22 01:03 PM, Jörn Engel wrote:
>>>>>>>>> On Mon, 22 July 2013 12:40:29 +0800, Vaughan Cao wrote:
>>>>>>>>>>
>>>>>>>>>> There is a race when open sg with O_EXCL flag. Also a race may
>>>>>>>>>> happen between
>>>>>>>>>> sg_open and sg_remove.
>>>>>>>>>>
>>>>>>>>>> Changes from v4:
>>>>>>>>>>      * [3/4] use ERR_PTR series instead of adding another
>>>>>>>>>> parameter in
>>>>>>>>>> sg_add_sfp
>>>>>>>>>>      * [4/4] fix conflict for cherry-pick from v3.
>>>>>>>>>>
>>>>>>>>>> Changes from v3:
>>>>>>>>>>      * release o_sem in sg_release(), not in sg_remove_sfp().
>>>>>>>>>>      * not set exclude with sfd_lock held.
>>>>>>>>>>
>>>>>>>>>> Vaughan Cao (4):
>>>>>>>>>>       [SCSI] sg: use rwsem to solve race during exclusive open
>>>>>>>>>>       [SCSI] sg: no need sg_open_exclusive_lock
>>>>>>>>>>       [SCSI] sg: checking sdp->detached isn't protected when
>>>>>>>>>> open
>>>>>>>>>>       [SCSI] sg: push file descriptor list locking down to
>>>>>>>>>> per-device
>>>>>>>>>>         locking
>>>>>>>>>>
>>>>>>>>>>      drivers/scsi/sg.c | 178
>>>>>>>>>> +++++++++++++++++++++++++-----------------------------
>>>>>>>>>>      1 file changed, 83 insertions(+), 95 deletions(-)
>>>>>>>>>
>>>>>>>>> Patchset looks good to me, although I didn't test it on hardware
>>>>>>>>> yet.
>>>>>>>>> Signed-off-by: Joern Engel <joern@...fs.org>
>>>>>>>>>
>>>>>>>>> James, care to pick this up?
>>>>>>>>
>>>>>>>> Acked-by: Douglas Gilbert <dgilbert@...erlog.com>
>>>>>>>>
>>>>>>>> Tested O_EXCL with multiple processes and threads; passed.
>>>>>>>> sg driver prior to this patch had "leaky" O_EXCL logic
>>>>>>>> according to the same test. Block device passed.
>>>>>>>>
>>>>>>>> James, could you clean this up:
>>>>>>>>       drivers/scsi/sg.c:242:6: warning: unused variable ‘res’
>>>>>>>> [-Wunused-variable]
>>>>>>>
>>>>>>> Further testing suggests this patch on the sg driver is
>>>>>>> broken, so I'll rescind my ack.
>>>>>>>
>>>>>>> The case it is broken for is when a device is opened
>>>>>>> without O_EXCL. Now if, while it is open, a second
>>>>>>> thread/process tries to open the same device O_EXCL
>>>>>>> then IMO the second open should fail with EBUSY.
>>>>>>>
>>>>>>> My testing shows that O_EXCL opens properly deflect
>>>>>>> other O_EXCL opens.
>>>>>> Hi  Doug,
>>>>>>
>>>>>> My test don't have this issue. The routine is something as below:
>>>>>>
>>>>>> I start three opens without O_EXCL, wait 30s each, and open with
>>>>>> O_EXCL|O_NONBLOCK, it failed with EBUSY.
>>>>>> And I also call myopen with/without O_EXCL many times in
>>>>>> background at
>>>>>> the same time, and the test is passed. I don't know why it failed in
>>>>>> your test.
>>>>>>
>>>>>> Usage: myopen [-e][-n][-d delay] -f file
>>>>>>          -e: exclude
>>>>>>          -n: nonblock
>>>>>>          -d: delay N seconds and then close.
>>>>>>
>>>>>> [root@...aowol5 16835013]# ./myopen  -f /dev/sg5 -d 30 &
>>>>>> [1] 3417
>>>>>> [root@...aowol5 16835013]# ./myopen  -f /dev/sg5 -d 30 &
>>>>>> [2] 3418
>>>>>> [root@...aowol5 16835013]# ./myopen  -f /dev/sg5 -d 30 &
>>>>>> [3] 3419
>>>>>> [root@...aowol5 16835013]# cat /proc/scsi/sg/debug
>>>>>> max_active_device=6(origin 1)
>>>>>>     def_reserved_size=32768
>>>>>>     >>> device=sg5 scsi5 chan=0 id=1 lun=0   em=0 sg_tablesize=55
>>>>>> excl=0
>>>>>>       FD(1): timeout=60000ms bufflen=32768 (res)sgat=1 low_dma=0
>>>>>>       cmd_q=0 f_packid=0 k_orphan=0 closed=0
>>>>>>         No requests active
>>>>>>       FD(2): timeout=60000ms bufflen=32768 (res)sgat=1 low_dma=0
>>>>>>       cmd_q=0 f_packid=0 k_orphan=0 closed=0
>>>>>>         No requests active
>>>>>>       FD(3): timeout=60000ms bufflen=32768 (res)sgat=1 low_dma=0
>>>>>>       cmd_q=0 f_packid=0 k_orphan=0 closed=0
>>>>>>         No requests active
>>>>>>
>>>>>> [root@...aowol5 16835013]# ./myopen -e -n  -f /dev/sg5 -d 30 &
>>>>>> [4] 3422
>>>>>> [3422:3351] /dev/sg5:exclude: Device or resource busy
>>>>>>
>>>>>> [4]+  Exit 1                  ./myopen -e -n -f /dev/sg5 -d 30
>>>>>>
>>>>>> [root@...aowol5 16835013]# cat /proc/scsi/sg/debug
>>>>>> max_active_device=6(origin 1)
>>>>>>     def_reserved_size=32768
>>>>>>     >>> device=sg5 scsi5 chan=0 id=1 lun=0   em=0 sg_tablesize=55
>>>>>> excl=0
>>>>>>       FD(1): timeout=60000ms bufflen=32768 (res)sgat=1 low_dma=0
>>>>>>       cmd_q=0 f_packid=0 k_orphan=0 closed=0
>>>>>>         No requests active
>>>>>>       FD(2): timeout=60000ms bufflen=32768 (res)sgat=1 low_dma=0
>>>>>>       cmd_q=0 f_packid=0 k_orphan=0 closed=0
>>>>>>         No requests active
>>>>>>       FD(3): timeout=60000ms bufflen=32768 (res)sgat=1 low_dma=0
>>>>>>       cmd_q=0 f_packid=0 k_orphan=0 closed=0
>>>>>>         No requests active
>>>>>> [root@...aowol5 16835013]# cat /proc/scsi/sg/debug
>>>>>> [1]   Done                    ./myopen -f /dev/sg5 -d 30
>>>>>> [2]-  Done                    ./myopen -f /dev/sg5 -d 30
>>>>>> [3]+  Done                    ./myopen -f /dev/sg5 -d 30
>>>>>>
>>>>>
>>>>> Hi,
>>>>> After the initial failures about 36 hours ago, retesting
>>>>> yesterday and today has not produced any unexpected
>>>>> failures. And I have been trying hard on lk 3.10.4 and
>>>>> lk 3.10.5 .
>>>>>
>>>>> My test program is a bit more intense than yours and can
>>>>> be found in the sg3_utils beta in the News section of this
>>>>> page:
>>>>>     http://sg.danny.cz/sg/
>>>>>
>>>>> It is in the examples directory, two variants called
>>>>> sg_tst_excl and sg_tst_excl2 . You will need a recent gcc
>>>>> compiler, IOW something that can compile c++11 . gcc 4.7.3
>>>>> in Ubuntu 13.04 only just manages, fedora 19 should do
>>>>> better with gcc 4.8.1 . The threading is implemented using
>>>>> pthreads so it should be reliable.
>>>>>
>>>>> Typically I run multiple instances (processes) and each has
>>>>> multiple threads. One instance can run '-x' which will cause
>>>>> its first thread not to use O_EXCL **. All my tests currently
>>>>> use O_NONBLOCK and that leads to lots of EBUSYs (sometimes
>>>>> in the billions).
>>>>>
>>>>> Doug Gilbert
>>>>>
>>>>>
>>>>> ** Using '-x' on two instances will cause an expected failure
>>>>>      so can be used as a control.
>>>>>
>>>> Hi Doug,
>>>>
>>>> Can I regard this as you ACK it again?
>>>
>>> Hi,
>>> I'd like you to test your setup with sg_tst_excl or sg_tst_excl2 .
>>> Since my last email, I have not seen any more failures with those
>>> tests on the patched sg driver but I did see a couple on
>>> /dev/sd* . With sg_tst_excl2, bsg devices can be used and since bsg
>>> accepts and ignores O_EXCL, it fails reliably.
>>>
>>> BTW I use scsi_debug with 'delay=0' for a pseudo device.
>>>
>>> Doug Gilbert
>> Hi Doug,
>>
>> I run test for sg and sd drivers with both sg_tst_excl and sg_tst_excl2
>> on kernel vanilla 3.10.9 with mypatches included on Fedora19 x86_64
>> baremachine.
>> * I've tried several times with different -w and -n setting, no failure
>> for sg driver found.
>> * It's easy to find failure on both patched and non-patched kernel for
>> sd driver with the following test command:
>>      ./sg_tst_excl -w 3 -n 2000 -t 16 -x $1 &
>>      ./sg_tst_excl -w 3 -n 2000 -t 16 $1 &
>>      ./sg_tst_excl -w 0 -n 2000 -t 16 $1 &
>>      ./sg_tst_excl -w -1 -n 2000 -t 16 $1 &
>>      ./sg_tst_excl -w -2 -n 2000 -t 16 $1 &
>> I think option '-w 0/-1/-2' is significant to trigger the failure, since
>> when I only use '-w >0', test usually passed.
>
> Hi,
> Thanks for testing that. For others trying to follow this,
> the usage message for that test utility is:
>
> Usage: sg_tst_excl [-b] [-l <lba>] [-n <n_per_thr>] [-t <num_thrs>]
>                    [-V] [-w <wait_ms>] [-x] <disk_device>
>   where
>     -b                block on open (def: O_NONBLOCK)
>     -l <lba>          logical block to increment (def: 1000)
>     -n <n_per_thr>    number of loops per thread (def: 200)
>     -t <num_thrs>     number of threads (def: 4)
>     -V                print version number then exit
>     -w <wait_ms>      >0: sleep_for(<wait_ms>); =0: yield(); -1: no
>                       wait; -2: sleep(0)  (def: 0)
>     -x                don't use O_EXCL on first thread (def: use
>                       O_EXCL on all threads)
>
> Test O_EXCL open flag with sg driver. Each open/close cycle with the
> O_EXCL flag does a double increment on lba (using its first 4 bytes).
>
> ----------------------
>
> The test is using O_EXCL as a lock so that if a 4 byte integer in
> a block starts out as even (and for a scsi_debug pseudo, blocks are
> zero filled) then a open,double_increment,close sequence should
> always see an even number after the open. Notice that it should be
> safe for one process to run with '-x' so that its first thread
> opens the device without the O_EXCL flag.
>
>
> Acked-by: Douglas Gilbert <dgilbert@...erlog.com>
>
> And I let you run with the sd driver quirk :-)
Thanks, I appreciate testing the quirk for sd.

Vaughan
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