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 -- To unsubscribe from this list: send the line "unsubscribe stgt" in the body of a message to majordomo at vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html |