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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

  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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