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Re: [ANNOUNCE] libblkio v0.1.0 preview release
From: |
Richard W.M. Jones |
Subject: |
Re: [ANNOUNCE] libblkio v0.1.0 preview release |
Date: |
Thu, 29 Apr 2021 16:31:41 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, Apr 29, 2021 at 04:08:22PM +0100, Daniel P. Berrangé wrote:
> On Thu, Apr 29, 2021 at 04:00:38PM +0100, Richard W.M. Jones wrote:
> > On Thu, Apr 29, 2021 at 03:41:29PM +0100, Stefan Hajnoczi wrote:
> > > On Thu, Apr 29, 2021 at 03:22:59PM +0100, Richard W.M. Jones wrote:
> > > > On Thu, Apr 29, 2021 at 03:05:50PM +0100, Stefan Hajnoczi wrote:
> > > > > The purpose of this preview release is to discuss both the API design
> > > > > and general direction of the project. API documentation is available
> > > > > here:
> > > > >
> > > > > https://gitlab.com/libblkio/libblkio/-/blob/v0.1.0/docs/blkio.rst
> > > >
> > > > libvirt originally, and now libnbd, keep a per-thread error message
> > > > (stored in thread-local storage). It's a lot nicer than having to
> > > > pass &errmsg to every function. You can just write:
> > > >
> > > > if (nbd_connect_tcp (nbd, "remote", "nbd") == -1) {
> > > > fprintf (stderr,
> > > > "failed to connect to remote server: %s (errno = %d)\n",
> > > > nbd_get_error (), nbd_get_errno ());
> > > > exit (EXIT_FAILURE);
> > > > }
> > > >
> > > > (https://libguestfs.org/libnbd.3.html#ERROR-HANDLING)
> > > >
> > > > It means you can extend the range of error information available in
> > > > future. Also you can return a 'const char *' and the application
> > > > doesn't have to worry about lifetimes, at least in the common case.
> > >
> > > Thanks for sharing the idea, I think it would work well for libblkio
> > > too.
> > >
> > > Do you ignore the dlclose(3) memory leak?
> >
> > I believe this mechanism in libnbd ensures there is no leak in the
> > ordinary shared library (not dlopen/dlclose) case:
> >
> > https://gitlab.com/nbdkit/libnbd/-/blob/f9257a9fdc68706a4079deb4ced61e1d468f28d6/lib/errors.c#L35
> >
> > However I'm not sure what happens in the dlopen case, so I'm going to
> > test that out now ...
>
> pthread_key destructors are a disaster waiting to happen with
> dlclose, if the dlclose happens while non-main threads are
> still running. When those threads exit, they'll run the
> destructor which points to a function that is no longer
> resident in memory.
>
> IOW if you have destrutors, then you need to make sure your
> library uses "-z nodelete" when linking, to turn dlclose()
> into a no-op.
I suspect letting the string leak is a better idea for libnbd.
Still trying to write a nice reproducer ..
Rich.
> commit 8e44e5593eb9b89fbc0b54fde15f130707a0d81e
> Author: Daniel P. Berrangé <berrange@redhat.com>
> Date: Thu Sep 1 17:57:06 2011 +0100
>
> Prevent crash from dlclose() of libvirt.so
>
> When libvirt calls virInitialize it creates a thread local
> for the virErrorPtr storage, and registers a callback to
> cleanup memory when a thread exits. When libvirt is dlclose()d
> or otherwise made non-resident, the callback function is
> removed from memory, but the thread local may still exist
> and if a thread later exists, it will invoke the callback
> and SEGV. There may also be other thread locals with callbacks
> pointing to libvirt code, so it is in general never safe to
> unload libvirt.so from memory once initialized.
>
> To allow dlclose() to succeed, but keep libvirt.so resident
> in memory, link with '-z nodelete'. This issue was first
> found with the libvirt CIM provider, but can potentially
> hit many of the dynamic language bindings which all ultimately
> involve dlopen() in some way, either on libvirt.so itself,
> or on the glue code for the binding which in turns links
> to libvirt
>
>
>
> Regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
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--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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Re: [ANNOUNCE] libblkio v0.1.0 preview release, Kevin Wolf, 2021/04/29