Re: [PATCH 4/4] xfs: fix the logspace waiting algorithm

[Christoph Hellwig <hch@infradead.org>]

On Thu, Dec 01, 2011 at 01:51:28PM -0600, Ben Myers wrote:
> Process A reads from the grant reserve head at 2641 (and there currently is enough space)
> Process B wakes at either 2646 or 2650, in xlog_reserveq_wait, locks, and reads from the grant reserve head (and currently there is enough space)
> Process B removes itself from the list 
> Process A reads from the reservq list and finds it to be empty
> Process A finds that there was enough space at 2646
> Process B returns from xlog_reserveq_wait, unlocks, grants space at 2656, returns
> Process A grants log space at 2656, and returns
> 
> AFAICS there is nothing that prevents these guys from granting the same
> space when you approach free_bytes >= need_bytes concurrently. 
> 
> This lockless stuff is always a mind job for me.  I'll take another look at
> some of the other aspects of the patch.  Even if it doesn't resolve my
> question about the lockless issue, it seems to resolve Chandra's race.

Indeed, I think we have this race.  Then again I I think we had
exactly the same one before, too.  The only way to fix it would be
to do a sort of double cmpxchg that only moves the grant head forward
if it's still in available space vs the tails lsn.

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