Re: [PATCH 8/9] xfs: introduce xfs_buf_submit[_wait]

[Dave Chinner <david@fromorbit.com>]

On Fri, Aug 15, 2014 at 10:35:58AM -0400, Brian Foster wrote:
> On Fri, Aug 15, 2014 at 04:39:06PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > There is a lot of cookie-cutter code that looks like:
> > 
> > 	if (shutdown)
> > 		handle buffer error
> > 	xfs_buf_iorequest(bp)
> > 	error = xfs_buf_iowait(bp)
> > 	if (error)
> > 		handle buffer error
> > 
> > spread through XFS. There's significant complexity now in
> > xfs_buf_iorequest() to specifically handle this sort of synchronous
> > IO pattern, but there's all sorts of nasty surprises in different
> > error handling code dependent on who owns the buffer references and
> > the locks.
> > 
> > Pull this pattern into a single helper, where we can hide all the
> > synchronous IO warts and hence make the error handling for all the
> > callers much saner. This removes the need for a special extra
> > reference to protect IO completion processing, as we can now hold a
> > single reference across dispatch and waiting, simplifying the sync
> > IO smeantics and error handling.
> > 
> > In doing this, also rename xfs_buf_iorequest to xfs_buf_submit and
> > make it explicitly handle on asynchronous IO. This forces all users
> > to be switched specifically to one interface or the other and
> > removes any ambiguity between how the interfaces are to be used. It
> > also means that xfs_buf_iowait() goes away.
> > 
> > For the special case of delwri buffer submission and waiting, we
> > don't need to issue IO synchronously at all. The second pass to cal
> > xfs_buf_iowait() can now just block on xfs_buf_lock() - the buffer
> > will be unlocked when the async IO is complete. This formalises a
> > sane the method of waiting for async IO - take an extra reference,
> > submit the IO, call xfs_buf_lock() when you want to wait for IO
> > completion. i.e.:
> > 
> > 	bp = xfs_buf_find();
> > 	xfs_buf_hold(bp);
> > 	bp->b_flags |= XBF_ASYNC;
> > 	xfs_buf_iosubmit(bp);
> > 	xfs_buf_lock(bp)
> > 	error = bp->b_error;
> > 	....
> > 	xfs_buf_relse(bp);
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> 
> On a quick look at submit_wait this looks pretty good. It actually
> implements the general model I've been looking for for sync I/O. E.g.,
> send the I/O, wait on synchronization, then check for errors. In other
> words, a pure synchronous mechanism. The refactoring and new helpers and
> whatnot are additional bonus and abstract it nicely.
> 
> I still have to take a closer look to review the actual code, but since
> we go and remove the additional sync I/O reference counting business,
> why do we even add that stuff early on? Can't we get from where the
> current code is to here in a more direct manner?

Simply because we need a fix that we can backport, and that fix is a
simple addition that does not significantly affect the rest of the
patchset...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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