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From Stefan Fuhrmann <stefan.fuhrm...@wandisco.com>
Subject Re: svn commit: r1589284 - in /subversion/trunk/subversion/libsvn_fs_fs: fs.c hotcopy.c hotcopy.h
Date Wed, 06 Aug 2014 20:33:11 GMT
On Mon, Jul 28, 2014 at 2:27 AM, Evgeny Kotkov <evgeny.kotkov@visualsvn.com>
wrote:

> Hi Stefan,
>
> I recently stumbled upon this changeset and I had some time to review it.
>  I
> think that the things should not be done this way.  As I see it, this
> change
> breaks one fundamental aspect of the non-incremental hotcopy, adds an ugly
> hack to the fs_hotcopy() layer and can lead to a potential deadlock between
> 'hotcopy' and 'pack' operations.  See my comments inline.
>

Thank you for the review. You are very welcome to
commit improvements.

> -  hotcopy_setup_shared_fs_data(src_fs, dst_fs);
> > -  SVN_ERR(svn_fs_fs__initialize_caches(dst_fs, pool));
> > +  /* FS creation is complete. Stamp it with a format file. */
> > +  SVN_ERR(svn_fs_fs__write_format(dst_fs, TRUE, pool));
> >
> >    return SVN_NO_ERROR;
> >  }
>
> The whole idea of the non-incremental hotcopy is that it *does not* make
> the
> destination visible until the hotcopy finishes.


No. The whole idea of hotcopy is that repository backups
can be created quickly (faster than dump / load) and
without disrupting r/w access.

Adding the ability to do that incrementally does not change
the semantics of existing usage. The API does not provide
any guarantees concerning transitional states, except maybe
that incremental hotcopy should recover from interrupted
previous hotcopies (not states explicitly but one might read
that into it).


>  This is how the things worked
> prior to this changeset


That behaviour was accidental.


> and this is how the svn_repos layer currently works.
> By writing the format file before actually doing something we do not only
> expose our internal stub to the world (which really is a stub without even
> r0),
>

Wrong. As you just said, svn_repos will not write the format
file until after the FS level operation succeeded. So, the only
part of the "world" that sees the empty repo is server-side
low-level scripts. Coordinating those is in the responsibility
of the respective administrator.


> but will also leave this internal stub openable in case the hotcopy fails.
> This stub does not even have to be a normal filesystem and we only need
> it to bootstrap the hotcopy process, but we now behave as if it was!
>

You are right that we would not be required by the API to have
a valid repository until the hotcopy succeeds. However, having
it is not break of the API contract. In particular, the repository
is always consistent, i.e. valid to be used from the FS API.

Even more so, since we copy data incrementally even without
the flag set, an aborted hotcopy process can be restarted / continued.


> Finally, the 'repos' and 'fs' layers now behave inconsistently in these
> terms,
> i.e. svn_repos_hotcopy3() still writes the format file only when
> everything is
> done.
>

This is basically correct, although no consistency requirement
has been violated. The reason why the repos layer delays
the accessibility is exactly your "don't become visible to the
world until done" feature. However, there is no guarantee for it.


> > +  SVN_ERR(svn_fs_fs__hotcopy_prepare_target(src_fs, dst_fs, dst_path,
> > +                                            incremental, pool));
> > +  dst_fs->fsap_data = NULL;
> > +
> > +  /* Now, the destination repo should open just fine. */
> > +  SVN_ERR(fs_open(dst_fs, dst_path, common_pool_lock, pool,
> common_pool));
>
> This is the hack I was talking about.  Clearing FSAP_DATA here is only
> required
> to pass the precondition check performed by svn_fs__check_fs() within the
> fs_open() call.


r1616338 should make it more clear what is going on here.
Have you looked at hotcopy_setup_shared_fs_data() in the
previous code? How is using the same locks for different
file systems not a hack or even safe?


>  By the way, how should an arbitrary caller use the introduced
> svn_fs_fs__hotcopy_prepare_target() function?  Should he/she *always* clear
> that field in order to be able to open the filesystem?
>

What arbitrary caller do you have in mind for this internal
function? The only reason to even have this private API
is the necessary interaction with code in fs.c.

>
> > +/* Wrapper around hotcopy_body taking out all necessary source
> repositories.
> > + */
> > +static svn_error_t *
> > +hotcopy_locking_src_body(void *baton, apr_pool_t *pool)
> >  {
> > -  fs_fs_data_t *src_ffd = src_fs->fsap_data;
> > -  fs_fs_data_t *dst_ffd = dst_fs->fsap_data;
> > +  struct hotcopy_body_baton *hbb = baton;
> > +  fs_fs_data_t *src_ffd = hbb->src_fs->fsap_data;
> >
> > -  /* The common pool and mutexes are shared between src and dst
> filesystems.
> > -   * During hotcopy we only grab the mutexes for the destination, so
> there
> > -   * is no risk of dead-lock. We don't write to the src filesystem.
> Shared
> > -   * data for the src_fs has already been initialised in fs_hotcopy().
> */
> > -  dst_ffd->shared = src_ffd->shared;
> > +  return src_ffd->format >= SVN_FS_FS__MIN_PACK_LOCK_FORMAT
> > +    ? svn_error_trace(svn_fs_fs__with_pack_lock(hbb->src_fs,
> hotcopy_body,
> > +                                                baton, pool))
> > +    : hotcopy_body(baton, pool);
> >  }
>
> Here is the potential deadlock.  Say, someone runs 'svnadmin freeze
> svnadmin
> hotcopy' and at the same moment 'svnadmin pack' is executed for the
> repository.
> Packing takes the pack-lock for the whole operation, but sometimes also
> needs
> to take a nested write-lock within the pack_shard() function.  This is what
> could happen:
>
>   1. freeze (takes the db/write-lock, inner 'hotcopy' still pending to
> execute)
>   2. pack (takes the db/pack-lock, blocks trying to acquire the
> db/write-lock)
>   3. hotcopy (blocks trying to acquire the db/pack-lock, **deadlock!**)
>

You are right. Thanks for reporting this. After some IRC discussion
with Philip, we decided that freeze() is at fault and should simply
take out the pack lock as well. Done in r1615351.


> Personally, I also find this hunk cryptic due to the usage of the
> non-obvious
> ternary operator and due to another added layer of the indirection (now
> svn_fs_fs__hotcopy() calls hotcopy_locking_src_body() calling
> hotcopy_body()).
>

I'm very fond of the functional programming style enabled
by the ternary operator. A matter of taste and opinions vary.

The locking pattern that we use wherever feasible is using a
wrapper function ensuring proper lock release even in case
of some previous error.

The comment is also misleading.  What exactly are the "source repositories"
> in
> this context?
>

Probably got distracted when writing this comment. It should read
"take out source repository locks". Fixed in r1615348.

-- Stefan^2.

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