Re: [PATCH v5 0/4] [SCSI] sg: fix race condition in sg_open

[Douglas Gilbert <dgilbert@interlog.com>]

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@logfs.org>
>>>>
>>>> James, care to pick this up?
>>>
>>> Acked-by: Douglas Gilbert <dgilbert@interlog.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@vacaowol5 16835013]# ./myopen  -f /dev/sg5 -d 30 &
> [1] 3417
> [root@vacaowol5 16835013]# ./myopen  -f /dev/sg5 -d 30 &
> [2] 3418
> [root@vacaowol5 16835013]# ./myopen  -f /dev/sg5 -d 30 &
> [3] 3419
> [root@vacaowol5 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@vacaowol5 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@vacaowol5 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@vacaowol5 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.

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