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Re: [Libguestfs] [libnbd PATCH] docs: Mention that nbd_close is not thread-safe

On 7/25/19 3:56 AM, Martin Kletzander wrote:
> On Thu, Jul 25, 2019 at 09:41:01AM +0100, Richard W.M. Jones wrote:
>> On Wed, Jul 24, 2019 at 03:14:28PM -0500, Eric Blake wrote:
>>> Closing the handle in one thread while another thread is locked causes
>>> undefined behavior (as closing does not obtain a lock, and can cause
>>> use-after-free or other nasty problems to the thread that does own the
>>> lock).  However, it is not sensible to try and obtain a lock in
>>> nbd_close, as POSIX says that it is also undefined for any other
>>> thread to wait on a mutex that has already been destroyed.  Therefore,
>>> we don't need to change our code, but merely remind users that
>>> nbd_close is not safe until all other uses of the handle have ceased.
>>> ---

>> Yes it's not safe to call nbd_close until all other uses of
>> the same handle from any other thread are over.
> Would it be too much of a trouble to add reference counting and give
> users a way
> to "copy" of the handle?  It wouldn't be a copy, but merely an increment
> on the
> reference counter.

So each time you hand an nbd handle to another thread, that new thread
has to increment the counter, and then every thread must call nbd_close
when done with their copy (but only the last thread to do so actually
frees resources).  It might help, but only if we are willing to go that

>  Or is it not worth doing that?

...and as Rich says, managing refcounting ourselves even with language
bindings is harder than it looks (how do you guarantee the refcount is
increased when another language has other ways to share the handle
between threads?).  The user can just as easily pthread_join before
calling nbd_close, at which point the problem is equally solved, but
with less burden on the library.  I don't think we've locked ourselves
in a corner - if there is a compelling reason to add thread-safe close
via ref-counting later on, we can do it, and update the docs at that
time, but for now I'm fine living with the doc patch.

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

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