[stgt] [PATCH 1/2] DLL backing store

[FUJITA Tomonori]

On Tue, 19 Aug 2008 14:52:23 +1000
Mark Harvey <markh794 at gmail.com> wrote:

> Richard Sharpe wrote:
> > On Sun, Aug 17, 2008 at 10:48 PM, Mark Harvey <markh794 at gmail.com> wrote:
> >> From ea4d57fca07516c03980970cf90b937c45e3811e Mon Sep 17 00:00:00 2001
> >> From: Mark Harvey <markh794 at gmail.com>
> >> Date: Mon, 18 Aug 2008 15:03:21 +1000
> >> Subject: New backing store for ssc type devices.
> >>
> >> Uses a double-linked list header with each block of data.
> >>
> >> Implement basic fixed block READ_6 & WRITE_6 OP codes.
> >>
> >> Still along way to go.
> >> - Race condition on blk header between threads.
> > 
> > Hmmm, why do you say there is a race here? Surely only one initiator
> > can access a tape at one time, and unless you have multiple threads
> > for the one tape, there should not be a problem here, unless I am
> > missing something?
> > 
> 
> It may be me that's missing something. I still have not got my head around the multi-threaded code.
> 
> However I assume that 'multi-threaded' allows a worker thread to accept one SCSI op code.
> 
> If the 2nd (or subsequent command) arrives during the update of the blk_header by the earlier thread, then there is a possibility of one thread using the blk_header belonging to the first thread and over writing each others data & header.
> 
> e.g.
> writeA -> read blk_headerX (and start to copy/build new blk_headerY)
> writeB arrives before writeA has completed building blk_headerY
> 
> Results is A & B writes will use the same blk_header (blk_headerX) information and over-write each others data.
> 
> I am more then happy to be told I'm wrong.

I've not looked at the code, but it sounds like you need to use
pthread_mutex_lock/unlock.

Using only one thread could be a quick fix for now.


> But I see some simple locking process during the period of 'read header -> update new header & save in SSC header'. Then allow actual write/read of any data process to complete outside the mutex/spinlock
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