linux-kernel - Re: [PATCH v2 1/1] [SCSI] sg: fix race condition when do exclusive open

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Date:	Sun, 07 Jul 2013 01:24:44 +0800
From:	vaughan <vaughan.cao@...cle.com>
To:	Jörn Engel <joern@...fs.org>
CC:	dgilbert@...erlog.com, JBottomley@...allels.com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	vaughan.cao@...cle.com
Subject: Re: [PATCH v2 1/1] [SCSI] sg: fix race condition when do exclusive
 open

On 07/06/2013 01:39 AM, Jörn Engel wrote:
> Sorry about replying so late.
>
> On Mon, 17 June 2013 21:10:53 +0800, vaughan wrote:
>> Rewrite the last patch.
>> Add a new field 'toopen' in sg_device to count ongoing sg_open's. By checking both 'toopen' and 'exclude' marks when do exclusive open, old race conditions can be avoided.
>> Replace global sg_open_exclusive_lock with a per device lock - sfd_lock. Since sfds list is now protected by the lock owned by the same sg_device, sg_index_lock becomes a real global lock to only protect sg devices lookup.
>> Also did some cleanup, such as remove get_exclude() and rename set_exclude() to clear_exclude().
>>
> ...
>> @@ -171,10 +168,10 @@ typedef struct sg_device { /* holds the state of each scsi generic device */
>>   	wait_queue_head_t o_excl_wait;	/* queue open() when O_EXCL in use */
>>   	int sg_tablesize;	/* adapter's max scatter-gather table size */
>>   	u32 index;		/* device index number */
>> -	/* sfds is protected by sg_index_lock */
>> +	spinlock_t sfd_lock;	/* protect sfds, exclude, toopen */
>>   	struct list_head sfds;
>> +	int toopen;		/* number of who are ready to open sg */
>                                              ^
> I think the 'toopen' is a bad choice.  I'm having trouble wrapping my
> head around the semantics of this variable, your description feels a
> bit handwavy, the main noun is missing in the command above, I think I
> found one more overflow bug,...
>
> What you ended up doing is reimplement a rw_semaphone.  Why not use
> one instead?  down_write() for exclusive access, down_read() for
> non-exclusive, _trylock variants for nonblocking opens, etc.
The critical part of open is to add a new sfd to the list and its 
protected by the
spin_lock(sg_index_lock previously) well. So I added an counter as a 
sign rather than
introducing another spinlock or mutex which means I should deal with 
potential deadlock.
The code may be simpler with a rwsem implementation as you suggest, I'll 
modify it in
this way.

There is no overflow bug, I eliminated it with the following line :)
      if (!sdp->exclude && sdp->toopen != INT_MAX) { ...

Do you agree that I use a per device spin_lock 'sfd_lock' to protect 
sfds list and leave sg_index_lock
only protect the global sg device lookup? I think it's reasonable for 
concurrency.


Thanks,
Vaughan

>
> Would this work?

>
> Jörn
>
> --
> I've never met a human being who would want to read 17,000 pages of
> documentation, and if there was, I'd kill him to get him out of the
> gene pool.
> -- Joseph Costello

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