[dm-devel] [PATCH 3/4] [block-manager] remove spurious decrement of write_lock_pending in the case of a recycled block.

[Joe Thornber]

On Wed, Aug 03, 2011 at 10:50:33AM -0400, Mikulas Patocka wrote:
> I think this is not correct.

I had a similar thought last night, however my concern was the
previous 'read' patch that you've acked.  I'll go back and look at
these today.

- Joe

> 
> The problem here is that the block may have been recycled and the newly 
> created block may have the same block number as the old block.
> 
> If b->where != block, we know that the block was recycled.
> If b->where == block, the block may have been recycled or not and we 
> don't know.
> 
> 
> I think the correct solution could be: make write_lock_pending a boolean 
> variable, not a counter.
> 
> Set write_lock_pending inside __wait_block when we are about to wait (the 
> block may have been recycled each time we waited, so we need to set it 
> each time we are going to wait)
> Clear write_lock_pending when __wait_unlocked exits.
> 
> If we make it a boolean variable, double clearing makes no harm.
> 
> Mikulas
> 
> On Tue, 2 Aug 2011, Joe Thornber wrote:
> 
> > ---
> >  drivers/md/persistent-data/dm-block-manager.c |    8 +++++++-
> >  1 files changed, 7 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/md/persistent-data/dm-block-manager.c b/drivers/md/persistent-data/dm-block-manager.c
> > index b68be88..d27ab6e 100644
> > --- a/drivers/md/persistent-data/dm-block-manager.c
> > +++ b/drivers/md/persistent-data/dm-block-manager.c
> > @@ -756,9 +756,15 @@ retry:
> >  
> >  				b->write_lock_pending++;
> >  				__wait_unlocked(b, &flags);
> > -				b->write_lock_pending--;
> >  				if (b->where != block)
> > +					/*
> > +					 * Recycled blocks have their
> > +					 * write_lock_pending count reset
> > +					 * to zero, so no need to undo the
> > +					 * above increment.
> > +					 */
> >  					goto retry;
> > +				b->write_lock_pending--;
> >  			}
> >  			break;
> >  		}
> > -- 
> > 1.7.4.1
> >