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Re: [PATCH] Fixing the FSFS corruption bug, again

From: Garrett Rooney <rooneg_at_electricjellyfish.net>
Date: 2006-10-03 15:06:05 CEST

On 10/2/06, Malcolm Rowe <malcolm-svn-dev@farside.org.uk> wrote:
> On Mon, Oct 02, 2006 at 05:24:38PM -0400, Garrett Rooney wrote:
> > This seems to work just fine on OS X and Linux, FWIW, but there's at
> > least one thing that worries me. The locking scheme used for managing
> > the per-txn stuff really could use some commenting, since I've been
> > staring at it for at least half an hour now trying to make sure I've
> > got it right.
> >
>
> Heh. As I mentioned on IRC, that was part of the code I'd thought I'd
> _over_ commented :-)
>
> The relevant comments are in libsvn_fs_fs/fs.h, where the structure is
> defined:
>
> /* A list of shared transaction objects for each transaction that is
> currently active, or NULL if none are. All access to this list,
> including the contents of the objects stored in it, is synchronised
> under TXN_LIST_LOCK. */
> fs_fs_shared_txn_data_t *txns;
>
> I thought that was pretty clear, but I'm probably too close to the
> problem; can anyone else suggest a better wording to that or any of the
> other comments?

Yeah, I just happened to miss that bit. Now that I've read it, I
understand totally.

> > Just to be clear, there's a single per-filesystem mutex that's used
> > within the with_txnlist_lock function to serialize access to both the
> > list of transactions and some of the data within the transactions. We
> > grab that lock, grab the flock, then note "hey, i've got it locked",
> > then drop the mutex. Later on when someone else tries to modify the
> > txn they'll be able to lock the per-filesystem mutex, but at that
> > point they'll see that someone else is modifying the txn, and error
> > out before they try to grab the flock? Other processes will simply
> > block when trying to flock.
> >
>
> s/some of the data/all of the data in that structure/, but otherwise,
> that's entirely correct. Also note that 'access' in the comment above
> means reading as well as writing.
>
> > I guess the part that confused me was how we've got a single per-fs
> > mutex, but per-txn lock files. In retrospect it makes sense, but it
> > was a bit weird at first. Other than that, this all seems fine, and I
> > think the confusion could be taken care of by some comments that sum
> > up the overall locking strategy, especially the fact that the
> > txn_list_lock ends up protecting both the list and the being_written
> > field.
> >
>
> Well, the way I was thinking was:
>
> - there's ->txn_list_lock, which is a mutex that protects all access to
> the txns list (and also the free-txn member).
> - there's txn->being_written, which is the intra-process lock on the
> transaction. It doesn't have to be a heavyweight mutex, since we
> serialise all access to the list.
> - finally, there's the transaction lockfiles themselves, which we
> lock if it's safe to attempt to do so.
>
> > Oh, and you've also got some mega-long lines in there that need to be
> > wrapped, but I'm sure you would have taken care of that before
> > committing.
> >
>
> Yes, I was going to ask: is it safe to split translated messages?
>
> _("That is, "
> "like this?")
>
> I was worried that doing so might break the translation tools.
>
>
> My only concern with the approach I've taken is the possibility of
> accreting a collection of transactions that have neither been committed
> nor aborted; these would lie around in memory in the same way they'd lie
> around on disk. On the other hand, that shouldn't be that common a
> problem, should it?

I'm not sure there's a real work around for it anyway.

-garrett

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Received on Tue Oct 3 15:06:34 2006

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