Re: [RESEND PATCH] writeback: Judge bdi->dev when set worker desc in bdi_writeback_workfn.

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On Thu, Sep 26, 2013 at 10:47:02AM -0700, Christoph Hellwig wrote:
> On Thu, Sep 26, 2013 at 08:43:51AM +1000, Dave Chinner wrote:
> > You're just papering over the larger problem, in that the writeback
> > work is running concurrently with the bdi_unregister() function that
> > is tearing the bdi down. You should try to fix the underlying race
> > condition, as documented in bdi_destroy.
> 
> The problem is even worse than that, and it's the lack of a proper
> refcounting on the bdi as seen by gems like bdi_prune_sb and the moving
> of writeback requests in bdi_destroy.  The right fix is to dynamically
> allocate the bdi (or at least the bdi_writeback), and make sure that we
> keep it around as long as a filesystem and thus the writeback code
> refers to.  Then a block device going away can just set a flag to stop
> writeback from trying instead of having the bdi ripped out underneath
> it which is guaranteed to fail and historically has failed in a large
> number of misterious ways.

Yup, be nice to see this rat-hole cleaned up properly, especially
those silly "pull disk, unplug-runs-sync_fs, fs hangs trying to do
IO to the disk that just got unplugged" bugs we get reported every
so often.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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