Re: Lockless Refs? (Was [PATCH] refs: do not use cached refs in repack_without_ref)
- From
- Martin Fick <mfick@codeaurora.org>
- Date
- Jan 4, 2013, 18:01 UTC
- Message-ID
- <201301041101.02756.mfick@codeaurora.org>
- In-Reply-To
- <871B6C10EBEFE342A772D1159D1320853A011469@umechphj.easf.csd.disa.mil>
On Friday, January 04, 2013 10:52:43 am Pyeron, Jason J CTR (US) wrote:
> > From: Martin Fick > > Sent: Thursday, January 03, 2013 6:53 PM > > > > Any thoughts on this idea? Is it flawed? I am
trying
> > to write it up in a more formal generalized manner
and
> > was hoping to get at least one "it seems sane"
before
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> > I do. > > If you are assuming that atomic renames, etc. are > available, then you should identify a test case and a > degrade operation path when it is not available.
Thanks, sound reasonable. Where you thinking a runtime test case that would be run before every transaction? I was anticipating a per repo config option called something like "core.locks = recoverable" that would be needed to turn them on? I was thinking that this was something that server sites could test in advance on their repos and then enable it for them. Maybe a git- lock tool with a --test-recoverable option?
-Martin
> > > > On Monday, December 31, 2012 03:30:53 am Martin Fick
wrote:
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> > > On Thursday, December 27, 2012 04:11:51 pm Martin > > > Fick > > > > wrote: > > > > It concerns me that git uses any locking at all, > > > > even for refs since it has the potential to
leave
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> > > > around stale locks. > > > > ... > > > > [a previous not so great attempt to fix this] > > > > ... > > > > > > I may have finally figured out a working loose ref > > > update mechanism which I think can avoid stale > > > locks. Unfortunately it requires atomic directory > > > renames and universally unique identifiers
(uuids).
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> > > These may be no-go criteria? But I figure it is > > > worth at least exploring this idea because of the > > > potential benefits? > > > > > > The general approach is to setup a transaction and > > > either commit or abort it. A transaction can be > > > setup by renaming an appropriately setup directory > > > to the "ref.lock" name. If the rename succeeds,
the
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> > > transaction is begun. Any actor can abort the > > > transaction (up until it is committed) by simply > > > deleting the "ref.lock" directory, so it is not at > > > risk of going stale. However, once the actor who > > > sets up the transaction commits it, deleting the > > > "ref.lock" directory simply aids in cleaning it up > > > for the next transaction (instead of aborting it). > > > > > > One important piece of the transaction is the use
of
> > > uuids. The uuids provide a mechanism to tie the > > > atomic commit pieces to the transactions and thus
to
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> > > prevent long sleeping process from inadvertently > > > performing actions which could be out of date when > > > they wake finally up. In each case, the atomic > > > commit piece is the renaming of a file. For the > > > create and update pieces, a file is renamed from
the
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> > > "ref.lock" dir to the "ref" file resulting in an > > > update to the sha for the ref. However, in the > > > delete case, the "ref" file is instead renamed to > > > end up in the "ref.lock" directory resulting in a > > > delete of the ref. This scheme does not affect
the
> > > way refs are read today, > > > > > > To prepare for a transaction, an actor first > > > generates a uuid (an exercise I will delay for
now).
> > > Next, a tmp directory named after the uuid is > > > generated in the parent directory for the ref to
be
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> > > updated, perhaps something like: ".lock_uuid". In > > > this directory is places either a file or a > > > directory named after the uuid, something like: > > > ".lock_uuid/,uuid". In the case of a create or an > > > update, the new sha is written to this file. In
the
> > > case of a delete, it is a directory. > > > > > > Once the tmp directory is setup, the initiating
actor
> > > attempts to start the transaction by renaming the
tmp
> > > directory to "ref.lock". If the rename fails, the > > > update fails. If the rename succeeds, the actor
can
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> > > then attempt to commit the transaction (before > > > another actor aborts it). > > > > > > In the case of a create, the actor verifies that > > > "ref" does not currently exist, and then renames
the
> > > now named "ref.lock/uuid" file to "ref". On
success,
> > > the ref was created. > > > > > > In the case of an update, the actor verifies that > > > "ref" currently contains the old sha, and then
also
> > > renames the now named "ref.lock/uuid" file to
"ref".
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> > > On success, the ref was updated. > > > > > > In the case of a delete, the actor may verify that > > > "ref" currently contains the sha to "prune" if it > > > needs to, and then renames the "ref" file to > > > "ref.lock/uuid/delete". On success, the ref was > > > deleted. > > > > > > Whether successful or not, the actor may now
simply
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> > > delete the "ref.lock" directory, clearing the way > > > for a new transaction. Any other actor may delete > > > this directory at any time also, likely either on > > > conflict (if they are attempting to initiate a > > > transaction), or after a grace period just to > > > cleanup the FS. Any actor may also safely cleanup > > > the tmp directories, preferably also after a grace > > > period. > > > > > > One neat part about this scheme is that I believe
it
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> > > would be backwards compatible with the current > > > locking mechanism since the transaction directory > > > will simply appear to be a lock to older clients. > > > And the old lock file should continue to lock out > > > these newer transactions. > > > > > > Due to this backwards compatibility, I believe
that
> > > this could be incrementally employed today without > > > affecting very much. It could be deployed in
place
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> > > of any updates which only hold ref.locks to update > > > the loose ref. So for example I think it could > > > replace step 4a below from Michael Haggerty's > > > description of today's loose ref pruning during > > > > > > ref packing: > > > > * Pack references: > > > ... > > > > > > > 4. prune_refs(): for each ref in the
ref_to_prune
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> > > > list, > > > > > > > > call prune_ref(): > > > > a. Lock the reference using lock_ref_sha1(), > > > > verifying that the recorded SHA1 is still > > > > valid. If it is, unlink the loose reference > > > > file then free the lock; otherwise leave the > > > > loose reference file untouched. > > > > > > I think it would also therefore be able to replace > > > the loose ref locking in Michael's new ref-packing > > > scheme as well as the locking in Michael's new ref > > > deletion scheme (again steps > > > > > > 4): > > > > * Delete reference foo: > > > ... > > > > > > > 4. Delete loose ref for "foo": > > > > a. Acquire the lock > > > > $GIT_DIR/refs/heads/foo.lock > > > > > > > > b. Unlink $GIT_DIR/refs/heads/foo if it is > > > > unchanged. > > > > > > > > If it is changed, leave it untouched. If it is > > > > deleted, > > > > > > > > that is OK too. > > > > > > > > c. Release lock
$GIT_DIR/refs/heads/foo.lock
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> > > > > > ... > > > > > > > * Pack references: > > > ... > > > > > > > 4. prune_refs(): for each ref in the
ref_to_prune
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> > > > list, > > > > > > > > call prune_ref(): > > > > a. Lock the loose reference using > > > > lock_ref_sha1(), > > > > > > > > verifying that the recorded SHA1 is still valid > > > > > > > > b. If it is, unlink the loose reference
file
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> > > > > > > > (otherwise, leave it untouched) > > > > > > > > c. Release the lock on the loose reference > > > > > > To be honest, I suspect I missed something obvious > > > because this seems almost too simple to work. I
am
> > > ashamed that it took me so long to come up with
(of
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> > > course, I will be even more ashamed :( when it is > > > shown to be flawed!) This scheme also feels > > > extensible. if there are no obvious flaws in it, I > > > will try to post solutions for ref packing and for > > > multiple repository/ref transactions also soon. > > > > > > I welcome any comments/criticisms, > > > > > > -Martin > > > -- > > > To unsubscribe from this list: send the line > > > "unsubscribe git" in the body of a message to > > > majordomo@vger.kernel.org More majordomo info at > > > http://vger.kernel.org/majordomo-info.html > > > > -- > > To unsubscribe from this list: send the line > > "unsubscribe git" in the body of a message to > > majordomo@vger.kernel.org More majordomo info at > > http://vger.kernel.org/majordomo-info.html
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