[Libguestfs] [nbdkit PATCH 8/8] rate: Atomically set CLOEXEC on fds

[Eric Blake]

On 8/1/19 4:12 AM, Richard W.M. Jones wrote:
> On Wed, Jul 31, 2019 at 05:01:52PM -0500, Eric Blake wrote:
>> On 7/31/19 4:31 PM, Eric Blake wrote:
>>> The rate filter is potentially opening fds in one thread while another
>>> thread is processing a fork() in the plugin.  Although the file is not
>>> open for long, we MUST atomically use CLOEXEC to avoid fd leaks.  This
>>> one is a bit harder to observe using only the sh plugin, because the
>>> window is small; you'll have better success at catching the leak by
>>> using gdb or recompiling code to insert strategic sleeps.
>>
>> In fact, I have to tweak this commit message: you CAN'T observe this one
>> with the sh plugin unless you recompile it to use #define THREAD_MODEL
>> NBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS, as well as introducing the
>> timing hacks mentioned above (that's because with our current
>> SERIALIZE_ALL_REQUESTS, there is never more than one thread in
>> filter/plugin code at a time).
> 
> The current nbdkit-sh-plugin is only SERIALIZE_ALL_REQUESTS in order
> to make writing the shell scripts a bit more sane.  I believe it could
> be fully PARALLEL.

Other than the fact that it uses pipe() instead of pipe2(), I'm not
seeing any other strong reasons why it can't be parallel.  I'll change
patch 9 along those lines.

> 
> (As an aside: Ideally in future we'll allow the thread model to be
> specified by the plugin dynamically.  It's one of the things I thought
> I had listed in the TODO file - it wasn't there so I've added it now.)

That's because we already have that: See commit afbcd070 and nearby.  So
I'll just revert your TODO change :)

> 
>> But it does raise an interesting point - if we hit platforms that are
>> unable to support atomic CLOEXEC, one possibility is a patch that forces
>> SERIALIZE_ALL_REQUESTS as the maximum parallelism allowed on that
>> platform (while remaining at our goal of PARALLEL on more competent
>> systems) - once we do that, the lacking systems will be serialized to
>> the point that there is no race window where one thread can fork() while
>> another is obtaining an fd.
> 
> Yup.  But probably better to encourage those platforms to support
> atomic CLOEXEC everywhere.

Yes, it would be nice for Haiku to realize how much they are losing out
on by not providing it.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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