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Date:      Wed, 13 Jun 2007 14:17:18 -0700 (PDT)
From:      youshi10@u.washington.edu
To:        hackers@freebsd.org
Subject:   Re: Using shell commands versus C equivalents
Message-ID:  <Pine.LNX.4.43.0706131417180.18410@hymn01.u.washington.edu>
In-Reply-To: <20070613182555.GA86571@keira.kiwi-computer.com>

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On Wed, 13 Jun 2007, Rick C. Petty wrote:

> On Wed, Jun 13, 2007 at 10:23:36AM -0700, youshi10@u.washington.edu wrote:
>> On Wed, 13 Jun 2007, Joerg Sonnenberger wrote:
>>
>> Should I briefly lock (flock) the file when running open/fstat/fchmod then
>> to avoid issues? This may become a problem as pkg_*/make becomes more
>> parallelized (another student's goals for his SoC project).
>
> I wouldn't bother.  What issue are you trying to avoid?  If one process is
> trying to chmod +x and another is trying to do a chmod -x, it shouldn't
> matter if you lock between the fstat/fchmod-- someone is going to win
> anyway.  This operation is not something that needs to be thread-safe.
>
>> Needless to say, pkg_* is by no means threadsafe in its current form
>> though. It uses some global vars that are currently not mutex locked, and
>> this type of file access is another issue (I wonder if spinlocking or
>> sleeping waiting for flock to finish would be better in this case).
>
> Does pkg_* use multiple threads?  I was under the impression each pkg tool
> used a single thread (i.e. no threads) to do its operations and that they
> wait for system(2)-type calls as needed.  Maybe I'm not clear by what you
> mean when you say "global vars".

Well, I mean that as it currently stands there are several variables used globally for setting attributes per package (I'm not at my machine right now so I can't look them up until ~6pm PST).

> Now another question is whether the pkg_* tools can handle multiple
> processes managing the ports at the same time.  For the mostpart, this is
> true.  Without looking at the code, I would expect that the only
> contentions would be when trying to update the +REQUIRED_BY files.
> Everything else should be just fine;  you're not supposed to be installing
> the same port multiple times at the exact same time, but maybe a lock could
> be held on the package directory (i.e. /var/db/pkg/$PKG_NAME).  Again, I
> don't believe this is strictly necessary.

Currently, no, and this is a condition that's contingent for a fellow SoC'er's project. The mentor said that all that *should* occur is there should be an flock, but that was it. So instead of making more work for him and since I am modifying pkg_* already, I thought it would be best to just make my modifications to simplify his end (he still has a ways to go on the dependency tracking I think).

It goes a bit deeper than the +REQUIRED_BY files, in particular with the +CONTENTS, etc files as the pkg_* tools are enumerating the packages currently on the system, their dependencies, owning files, etc. Perhaps a global .lock file of some kind in the package directories would be the way to go though.

Thanks,
-Garrett




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