linux-kernel - Re: [PATCH v5 0/4] [SCSI] sg: fix race condition in sg

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Date:	Sat, 03 Aug 2013 01:25:29 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Jörn Engel <joern@...fs.org>,
	Vaughan Cao <vaughan.cao@...cle.com>
CC:	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 13-08-01 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.

BTW the standard block driver (e.g. /dev/sdc) is broken
in exactly the same way, according to my tests.

Doug Gilbert


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