From: Brandon Williams <bmwill@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Wong <e@80x24.org>, Jeff King <peff@peff.net>,
Jonathan Nieder <jrnieder@gmail.com>,
Eric Rannaud <eric.rannaud@gmail.com>,
Git Mailing List <git@vger.kernel.org>,
Johannes Schindelin <johannes.schindelin@gmx.de>,
Jeremy Serror <jeremy.serror@gmail.com>
Subject: Re: [TANGENT] run-command: use vfork instead of fork
Date: Tue, 16 May 2017 14:11:25 -0700 [thread overview]
Message-ID: <20170516211125.GA74302@google.com> (raw)
In-Reply-To: <CA+55aFygKMJH3BRitQftmtCTx6es93CTR4_y1ULGGE2rSsMyog@mail.gmail.com>
On 05/16, Linus Torvalds wrote:
> On Tue, May 16, 2017 at 12:35 PM, Eric Wong <e@80x24.org> wrote:
> >
> > Fwiw, most of the vfork preparation was already done by Brandon
> > and myself a few weeks ago, and cooking in pu.
>
> Oh, interesting. Was that done for vfork(), or is it for something
> else? Some of the changes seem almost overly careful. Is this for
> prep-work porting to some odd environment that doesn't really have a
> MMU at all? There's nothing fundamentally wrong with allocating memory
> after fork().
>
> But yes, it looks like it helps the vfork case.
>
> Linus
I started working on the run-command code when I ran into a deadlock in
'git grep --recurse-submodules'. When I added support for submodules to
grep I just assumed that launching a process (which atm is unfortunately
the only way to work on a submodule) would work in a multi-threaded
environment. I was naive and wrong!
The deadlock was due to a malloc lock being held by thread 'A' while
thread 'b' tried to launch a process. Since that lock was in a
locked-state at the time of forking, it remained in a locked-state with
no hope of ever being released. So when the child process that thread
'b' spawned tried to malloc a chunk of memory after forking, it
deadlocked.
I didn't catch this in initial testing because gclib registers
atfork_handelers in order to prevent this sort of thing, while libraries
like tcmalloc don't do this.
So to account for this, I worked to make run-command safe to use in the
presence of threads, which had the benefit of also preparing it to be
vfork() ready.
Ultimately I'd like to drop the requirement to spawn a child process to
work on a submodule, but that's going to take a lot more effort.
--
Brandon Williams
next prev parent reply other threads:[~2017-05-16 21:11 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-15 18:08 git rebase regression: cannot pass a shell expression directly to --exec Eric Rannaud
2017-05-16 3:25 ` Jeff King
2017-05-16 3:37 ` Jeff King
2017-05-16 16:41 ` Jonathan Nieder
2017-05-16 16:47 ` Jeff King
2017-05-16 17:11 ` Eric Rannaud
2017-05-16 17:15 ` Brandon Williams
2017-05-16 17:23 ` Jeff King
2017-05-16 17:30 ` Brandon Williams
2017-05-16 19:14 ` Linus Torvalds
2017-05-16 19:35 ` [TANGENT] run-command: use vfork instead of fork Eric Wong
2017-05-16 20:47 ` Linus Torvalds
2017-05-16 21:11 ` Brandon Williams [this message]
2017-05-16 20:12 ` git rebase regression: cannot pass a shell expression directly to --exec Johannes Schindelin
2017-05-16 20:27 ` Linus Torvalds
2017-05-16 17:37 ` Jonathan Nieder
2017-05-16 3:40 ` Junio C Hamano
2017-05-16 3:53 ` Jeff King
2017-05-16 4:08 ` Jeff King
2017-05-16 16:45 ` Eric Rannaud
2017-05-16 10:46 ` Johannes Schindelin
2017-05-16 10:23 ` Johannes Schindelin
2017-05-16 16:18 ` Jeff King
2017-05-16 16:59 ` Eric Rannaud
2017-05-16 17:14 ` Kevin Daudt
2017-05-16 17:29 ` Eric Rannaud
2017-05-16 17:41 ` Jeff King
2017-05-16 17:21 ` Eric Rannaud
2017-05-16 17:37 ` Jeff King
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