git@vger.kernel.org mailing list mirror (one of many)
 help / Atom feed
From: Michael Haggerty <mhagger@alum.mit.edu>
To: Jeff King <peff@peff.net>, Brandon Williams <bmwill@google.com>
Cc: Lars Schneider <larsxschneider@gmail.com>,
	Martin Ågren <martin.agren@gmail.com>,
	Git Users <git@vger.kernel.org>
Subject: Re: [PATCH] config: use a static lock_file struct
Date: Wed, 30 Aug 2017 05:25:18 +0200
Message-ID: <01375356-5d39-99af-9e91-35083ed03f42@alum.mit.edu> (raw)
In-Reply-To: <20170829191217.dt65wazf7qh5qs3k@sigill.intra.peff.net>

On 08/29/2017 09:12 PM, Jeff King wrote:
> On Tue, Aug 29, 2017 at 12:09:28PM -0700, Brandon Williams wrote:
> 
>>> -- >8 --
>>> Subject: [PATCH] config: use a static lock_file struct
>>>
>>> When modifying git config, we xcalloc() a struct lock_file
>>> but never free it. This is necessary because the tempfile
>>> code (upon which the locking code is built) requires that
>>> the resulting struct remain valid through the life of the
>>> program. However, it also confuses leak-checkers like
>>> valgrind because only the inner "struct tempfile" is still
>>> reachable; no pointer to the outer lock_file is kept.
>>
>> Is this just due to a limitation in the tempfile code?  Would it be
>> possible to improve the tempfile code such that we don't need to require
>> that a tempfile, once created, is required to exist for the remaining
>> life of the program?
> 
> Yes. Like I wrote below:
> 
>>> ---
>>> In the long run we may want to drop the "tempfiles must remain forever"
>>> rule. This is certainly not the first time it has caused confusion or
>>> leaks. And I don't think it's a fundamental issue, just the way the code
>>> is written. But in the interim, this fix is probably worth doing.
> 
> The main issue is that the caller needs to make sure they're removed
> from the list (via commit or rollback) before being freed.
> 
> As far as I know anyway. This restriction dates back to the very early
> days of the lockfile code and has been carried through the various
> tempfile-cleanup refactorings over the years (mostly because each was
> afraid to make functional changes).
> 
> +cc Michael, who did most comprehensive cleanup of that code.

It was surprisingly hard trying to get that code to do the right thing,
non-racily, in every error path. Since `remove_tempfiles()` can be
called any time (even from a signal handler), the linked list of
`tempfile` objects has to be kept valid at all times but can't use
mutexes. I didn't have the energy to keep going and make the lock
objects freeable.

I suppose the task could be made a bit easier by using `sigprocmask(2)`
or `pthread_sigmask(3)` to make its running environment a little bit
less hostile.

Michael

  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-27  7:37 [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-27 15:41 ` Jeff King
2017-08-27 18:25   ` Jeff King
2017-08-27 18:21 ` Lars Schneider
2017-08-27 19:09   ` Martin Ågren
2017-08-27 19:15     ` Jeff King
2017-08-27 20:04     ` Lars Schneider
2017-08-27 23:23       ` Jeff King
2017-08-28  4:11         ` Martin Ågren
2017-08-28 17:52           ` Stefan Beller
2017-08-28 17:58             ` Jeff King
2017-09-05 10:03               ` Junio C Hamano
2017-08-29 17:51           ` Lars Schneider
2017-08-29 18:53             ` Jeff King
2017-08-29 18:58               ` [PATCH] config: use a static lock_file struct Jeff King
2017-08-29 19:09                 ` Brandon Williams
2017-08-29 19:10                   ` Brandon Williams
2017-08-29 19:12                   ` Jeff King
2017-08-30  3:25                     ` Michael Haggerty [this message]
2017-08-30  4:31                       ` Jeff King
2017-08-30  4:55                         ` Michael Haggerty
2017-08-30  4:55                         ` Jeff King
2017-08-30  5:55                           ` Jeff King
2017-08-30  7:07                             ` Michael Haggerty
2017-09-02  8:44                               ` Jeff King
2017-09-02 13:50                                 ` Jeff King
2017-08-30  6:55                           ` Michael Haggerty
2017-08-30 19:53                             ` Jeff King
2017-08-30 19:57                               ` Brandon Williams
2017-08-30 20:11                                 ` Jeff King
2017-08-30 21:06                                   ` Brandon Williams
2017-08-31  4:09                                     ` Jeff King
2017-09-06  3:59                 ` Junio C Hamano
2017-09-06 12:41                   ` Jeff King
2017-08-29 19:22               ` [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-29 21:48                 ` Jeff King
2017-08-30  5:31                   ` Jeff King
2017-09-05 10:03                     ` Junio C Hamano
2017-10-10  4:06                       ` [PATCH 0/2] Do not call cmd_*() as a subroutine Junio C Hamano
2017-10-10  4:06                         ` [PATCH 1/2] describe: do not use " Junio C Hamano
2017-10-10  4:06                         ` [PATCH 2/2] merge-ours: " Junio C Hamano

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=01375356-5d39-99af-9e91-35083ed03f42@alum.mit.edu \
    --to=mhagger@alum.mit.edu \
    --cc=bmwill@google.com \
    --cc=git@vger.kernel.org \
    --cc=larsxschneider@gmail.com \
    --cc=martin.agren@gmail.com \
    --cc=peff@peff.net \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

git@vger.kernel.org mailing list mirror (one of many)

Archives are clonable:
	git clone --mirror https://public-inbox.org/git
	git clone --mirror http://ou63pmih66umazou.onion/git
	git clone --mirror http://czquwvybam4bgbro.onion/git
	git clone --mirror http://hjrcffqmbrq6wope.onion/git

Newsgroups are available over NNTP:
	nntp://news.public-inbox.org/inbox.comp.version-control.git
	nntp://ou63pmih66umazou.onion/inbox.comp.version-control.git
	nntp://czquwvybam4bgbro.onion/inbox.comp.version-control.git
	nntp://hjrcffqmbrq6wope.onion/inbox.comp.version-control.git
	nntp://news.gmane.org/gmane.comp.version-control.git

 note: .onion URLs require Tor: https://www.torproject.org/
       or Tor2web: https://www.tor2web.org/

AGPL code for this site: git clone https://public-inbox.org/ public-inbox