Message-ID: <5209A50E.8090300@interlog.com>
Date:	Mon, 12 Aug 2013 23:16:30 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	vaughan <vaughan.cao@...cle.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 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


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