[PATCH 1/9] xfs: synchronous buffer IO needs a reference

[Dave Chinner]

On Fri, Aug 15, 2014 at 09:18:04AM -0400, Brian Foster wrote:
> On Fri, Aug 15, 2014 at 04:38:59PM +1000, Dave Chinner wrote:
....
> >  	if (bp->b_flags & XBF_WRITE)
> >  		xfs_buf_wait_unpin(bp);
> > +
> > +	/*
> > +	 * Take references to the buffer. For XBF_ASYNC buffers, holding a
> > +	 * reference for as long as submission takes is all that is necessary
> > +	 * here. The IO inherits the lock and hold count from the submitter,
> > +	 * and these are release during IO completion processing. Taking a hold
> > +	 * over submission ensures that the buffer is not freed until we have
> > +	 * completed all processing, regardless of when IO errors occur or are
> > +	 * reported.
> > +	 *
> > +	 * However, for synchronous IO, the IO does not inherit the submitters
> > +	 * reference count, nor the buffer lock. Hence we need to take an extra
> > +	 * reference to the buffer for the for the IO context so that we can
> > +	 * guarantee the buffer is not freed until all IO completion processing
> > +	 * is done. Otherwise the caller can drop their reference while the IO
> > +	 * is still in progress and hence trigger a use-after-free situation.
> > +	 */
> >  	xfs_buf_hold(bp);
> > +	if (!(bp->b_flags & XBF_ASYNC))
> > +		xfs_buf_hold(bp);
> > +
> >  
> >  	/*
> > -	 * Set the count to 1 initially, this will stop an I/O
> > -	 * completion callout which happens before we have started
> > -	 * all the I/O from calling xfs_buf_ioend too early.
> > +	 * Set the count to 1 initially, this will stop an I/O completion
> > +	 * callout which happens before we have started all the I/O from calling
> > +	 * xfs_buf_ioend too early.
> >  	 */
> >  	atomic_set(&bp->b_io_remaining, 1);
> >  	_xfs_buf_ioapply(bp);
> > +
> >  	/*
> > -	 * If _xfs_buf_ioapply failed, we'll get back here with
> > -	 * only the reference we took above.  _xfs_buf_ioend will
> > -	 * drop it to zero, so we'd better not queue it for later,
> > -	 * or we'll free it before it's done.
> > +	 * If _xfs_buf_ioapply failed or we are doing synchronous IO that
> > +	 * completes extremely quickly, we can get back here with only the IO
> > +	 * reference we took above.  _xfs_buf_ioend will drop it to zero, so
> > +	 * we'd better run completion processing synchronously so that the we
> > +	 * don't return to the caller with completion still pending. In the
> > +	 * error case, this allows the caller to check b_error safely without
> > +	 * waiting, and in the synchronous IO case it avoids unnecessary context
> > +	 * switches an latency for high-peformance devices.
> >  	 */
> 
> AFAICT there is no real wait if the buf has completed at this point. The
> wait just decrements the completion counter.

If the IO has completed, then we run the completion code.

> So what's the benefit of
> "not waiting?" Where is the potential context switch?

async work for completion processing on synchrnous IO means we queue
the work, then sleep in xfs_buf_iowait(). Two context switches, plus
a work queue execution

> Are you referring
> to the case where error is set but I/O is not complete? Are you saying
> the advantage to the caller is it doesn't have to care about the state
> of further I/O once it has been determined at least one error has
> occurred? (If so, who cares about latency given that some operation that
> depends on this I/O is already doomed to fail?).

No, you're reading *way* too much into this. For sync IO, it's
always best to process completion inline. For async, it doesn't
matter, but if there's a submission error is *more effecient* to
process it in the current context.

> The code looks fine, but I'm trying to understand the reasoning better
> (and I suspect we can clarify the comment).
> 
> > -	_xfs_buf_ioend(bp, bp->b_error ? 0 : 1);
> > +	if (bp->b_error || !(bp->b_flags & XBF_ASYNC))
> > +		_xfs_buf_ioend(bp, 0);
> > +	else
> > +		_xfs_buf_ioend(bp, 1);
> 
> Not related to this patch, but it seems like the problem this code tries
> to address is still possible.

The race condition is still possible - it just won't result in a
use-after-free. The race condition is not fixed until patch 8,
but as a backportable fix, this patch is much, much simpler.

> Perhaps this papers over a particular
> instance. Consider the case where an I/O fails immediately after this
> call completes, but not before. We have an extra reference now for
> completion, but we can still return to the caller with completion
> pending. I suppose its fine if we consider the "problem" to be that the
> reference goes away underneath the completion, as opposed to the caller
> caring about the status of completion.

Precisely.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com