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From: Michael Haggerty <mhagger@alum.mit.edu>
To: Jeff King <peff@peff.net>
Cc: Brandon Williams <bmwill@google.com>, 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 06:55:04 +0200
Message-ID: <CAMy9T_Eq+XJyhXk99RA9AMUVx3L8qw+0=KfAOjYsPsTSVSLchQ@mail.gmail.com> (raw)
In-Reply-To: <20170830043147.culn63luzdsbpuuw@sigill.intra.peff.net>

On Wed, Aug 30, 2017 at 6:31 AM, Jeff King <peff@peff.net> wrote:
> On Wed, Aug 30, 2017 at 05:25:18AM +0200, Michael Haggerty wrote:
>> 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.
>
> I think there are really two levels of carefulness here:
>
>   1. Avoiding complicated things during a signal handler that may rely
>      on having a sane state from the rest of the program (e.g.,
>      half-formed entries, stdio locks, etc).
>
>   2. Being truly race-free in the face of a signal arriving while we're
>      running arbitrary code that might have a tempfile struct in a funny
>      state.
>
> I feel like right now we meet (1) and not (2). But I think if we keep to
> that lower bar of (1), it might not be that bad. We're assuming now that
> there's no race on the tempfile->active flag, for instance. We could
> probably make a similar assumption about putting items onto or taking
> them off of a linked list (it's not really atomic, but a single pointer
> assignment is probably "atomic enough" for our purposes).
>
> Or I dunno. There's a lot of "volatile" modifiers sprinkled around.
> Maybe those are enough to give us (2) as well (though in that case, I
> think we'd still be as OK with the list manipulation as we are with the
> active flag manipulation).

I did in fact strive for both (1) and (2), though I know that there
are a couple of short races remaining in the code (plus who knows how
many bugs).

Actually, I think you're right that a single "probably-atomic" pointer
assignment would be enough to remove an entry from the list safely. I
was thinking about what it would take to make the list thread-safe
(which it currently is not, but probably doesn't need to be(?)).
Modifying the linked list is not that difficult if you assume that
there is only a single thread modifying the list at any given time.

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
2017-08-30  4:31                       ` Jeff King
2017-08-30  4:55                         ` Michael Haggerty [this message]
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

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