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

Kevin Corry
2002-12-11T14:44:46Z

On Wednesday 11 December 2002 13:24, Denis Vlasenko wrote:
> On 11 December 2002 12:18, Joe Thornber wrote:
> > On Wed, Dec 11, 2002 at 07:16:53AM -0600, Kevin Corry wrote:
> > > However, it might be a good idea to consider how bio's keep track
> > > of errors. When a bio is created, it is marked UPTODATE. Then, if
> > > any part of a bio takes an error, the UPTODATE flag is turned off.
> > > When the whole bio completes, if the UPTODATE flag is still on,
> > > there were no errors during the i/o. Perhaps the "error" field in
> > > "struct dm_io" could be modified to use this method of error
> > > tracking? Then we could change dec_pending() to be something like:
> > >
> > > if (error)
> > > clear_bit(DM_IO_UPTODATE, &io->error);
> > >
> > > with a "set_bit(DM_IO_UPTODATE, &ci.io->error);" in __bio_split().
> >
> > The problem with this is you don't keep track of the specific error
> > to later pass to bio_endio(io->bio...). I guess it all comes down to
> > just how expensive that spin lock is; and since locking only occurs
> > when there's an error I'm happy with things as they are.
>
> lock();
> a = b;
> unlock();
>
> Store of ints is atomic anyway. You need locking if a is a larger entity,
> say, a struct.

Storing an int is *not* atomic unless it is declared as atomic_t and you use
the appropriate macros (see include/asm-*/atomic.h). Remember, we are talking
about a field in a data structure that can be accessed from multiple threads
on multiple CPUs.

--
Kevin Corry
corryk@us.ibm.com