Re: Race between __sync_single_inode() and LogFS garbage collector

["Jörn Engel" <joern@lazybastard.org>]

On Mon, 19 February 2007 17:05:55 -0600, Dave Kleikamp wrote:
> 
> It'd be nice if you could drop s_w_mutex when the garbage collector
> calls i_get().

Not possible.  Garbage collection only happens when space is getting
tight.  At that moment all writes must be serialized or this race will
be the least of my problems. :(

> Otherwise, you may be able to call ilookup5_nowait() in the garbage
> collector, and skip that inode if I_LOCK is set.

Also not possible.  I cannot skip that inode, or again this race will be
a minor problem.  The inode exists on the medium and I must get it by
some means.  Re-reading it from the medium is fine, writing is not and
waiting for the write to happen brings me back to square one.

It is a nasty problem that has been haunting me for about a year now.
For a while I tried ilookup5_nowait() and just used the inode in spite
of the lock.  But that will explode spectacularly when racing against
generic_drop_inode().

Double-caching or a common lock seem to be the only solutions.

Jörn

-- 
My second remark is that our intellectual powers are rather geared to
master static relations and that our powers to visualize processes
evolving in time are relatively poorly developed.
-- Edsger W. Dijkstra
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