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From: Kevin Corry <corryk@us.ibm.com>
Organization: IBM
To: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>,
       Joe Thornber <joe@fib011235813.fsnet.co.uk>
Subject: Re: [PATCH] dm.c - device-mapper I/O path fixes
Date: Wed, 11 Dec 2002 08:02:24 -0600
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On Wednesday 11 December 2002 13:19, Denis Vlasenko wrote:
> On 11 December 2002 11:16, Kevin Corry wrote:
> > > > --- diff/drivers/md/dm.c	2002-12-11 12:00:29.000000000 +0000
> > > > +++ source/drivers/md/dm.c	2002-12-11 12:00:34.000000000 +0000
> > > > @@ -238,10 +238,11 @@
> > > >  	static spinlock_t _uptodate_lock = SPIN_LOCK_UNLOCKED;
> > > >  	unsigned long flags;
> > > >
> > > > -	spin_lock_irqsave(&_uptodate_lock, flags);
> > > > -	if (error)
> > > > +	if (error) {
> > > > +		spin_lock_irqsave(&_uptodate_lock, flags);
> > > >  		io->error = error;
> > > > -	spin_unlock_irqrestore(&_uptodate_lock, flags);
> > > > +		spin_unlock_irqrestore(&_uptodate_lock, flags);
> > > > +	}
> > > >
> > > >  	if (atomic_dec_and_test(&io->io_count)) {
> > > >  		if (atomic_dec_and_test(&io->md->pending))
> > >
> > > This seems pointless, end result:
> > >
> > > 	spin_lock_irqsave(&_uptodate_lock, flags);
> > >  	io->error = error;
> > > 	spin_unlock_irqrestore(&_uptodate_lock, flags);
> >
> > Are you saying the "if (error)" part is pointless? If so, I have to
>
> No. Locking is pointless. What exactly you try to protect here?

The "struct dm_io *io" that is passed to dec_pending() can be accessed by 
multiple threads at the same time, thus some form of locking is required.

I had been thinking about whether the "error" field could be an atomic_t, 
which would remove the requirement for the spinlock in dec_pending(). 
However, I don't know how atomic_t's behave with negative values. I know 
atomic_t's are only guaranteed to have 24-bits of precision, yet all arch's 
define atomic_t with a signed integer. Can anyone enlighten me on this?

Perhaps we could make "error" and atomic_t, and store the absolute-value of 
the error code, and always return -error in the bio_endio() call. Or is that 
just too ugly?

-- 
Kevin Corry
corryk@us.ibm.com
http://evms.sourceforge.net/
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