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

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