Re: [PATCH] dm.c - device-mapper I/O path fixes

Kevin Corry
2002-12-11T14:41:24Z

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