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Re: [Cluster-devel] [PATCH 4 of 5] Bz #248176: GFS2: invalid metadata block - REVISED


On Thu, 2007-08-09 at 14:21 -0400, Wendy Cheng wrote:
> Bob Peterson wrote:
> > On Thu, 2007-08-09 at 09:46 -0400, Wendy Cheng wrote:
> >   
> >> Set aside "after this patch, the problem goes away" thing ...
> >>
> >> I haven't checked previous three patches yet so I may not have the 
> >> overall picture ... but why adding the journal flush spin lock here 
> >> could prevent the new inode to get re-used before its associated buffer 
> >> are flushed to the logs ? Could you elaborate more ?
> >>
> >>     
> >>> +		down_write(&sdp->sd_log_flush_lock);
> >>>  		block = rgblk_search(rgd, goal, GFS2_BLKST_UNLINKED,
> >>>  				     GFS2_BLKST_UNLINKED);
> >>> +		up_write(&sdp->sd_log_flush_lock);
> >>>       
> >
> > IIRC, if we don't protect rgblk_search from finding GFS2_BLKST_UNLINKED
> > blocks, a "deleted" inode may be returned to function
> > gfs2_inplace_reserve_i which will do an iput on the inode,
> > which may reference buffers that are being flushed to disk.
> > If almost all blocks in that bitmap are allocated, I think the
> > deleted block may sometimes be reused and the buffer 
> > associated with the reused block may be changed before it's
> > actually written out to disk.
> >   
> Log flushing is an asynchronous event. I still don't see how this can 
> *protect* the condition you just described (i.e., prevents the block 
> being assigned to someone else before log flush occurs).  Or do I 
> understand your statement right (i.e., the log flushing must occur 
> before the block is used by someone else) ? It may *reduce* the 
> possibility (if log flushing happens at the same time as this 
> assignment) but I don't see how it can *stop* the condition.
> You may "reduce" the (rare) possibility but the real issue is still 
> hanging there ? If this is true, then I don't agree we have to pay the 
> price of moving a journal flushing lock into resource handling code.
> -- Wendy

Due to the way in which the locking is defined, the journal lock is also
used to keep other processes out of the rgrp bitmaps. This prevents the
state of the rgrp bitmaps changing while we are scanning them in case a
journal flush might occur.

The sd_log_flush_lock is an rwsem which is held in read mode by each and
every transaction and in write mode when flushing the journal. Log
flushing ought to be an asynchronous event, but due to the design of the
journaling, it unfortunately isn't in GFS2. It is something that we need
to review in the future,


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