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Re: [Qemu-devel] [PATCH v4 08/27] osdep: Add qemu_lock_fd and qemu_unloc

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] [PATCH v4 08/27] osdep: Add qemu_lock_fd and qemu_unlock_fd
Date: Tue, 10 May 2016 10:06:35 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

On Tue, May 10, 2016 at 09:57:48AM +0100, Daniel P. Berrange wrote:
> On Tue, May 10, 2016 at 10:50:40AM +0800, Fam Zheng wrote:
> > They are wrappers of POSIX fcntl file locking, with the additional
> > interception of open/close (through qemu_open and qemu_close) to offer a
> > better semantics that preserves the locks across multiple life cycles of
> > different fds on the same file.  The reason to make this semantics
> > change over the fcntl locks is to make the API cleaner for QEMU
> > subsystems.
> > 
> > More precisely, we remove this "feature" of fcntl lock:
> > 
> >     If a process closes any file descriptor referring to a file, then
> >     all of the process's locks on that file are released, regardless of
> >     the file descriptor(s) on which the locks were obtained.
> > 
> > as long as the fd is always open/closed through qemu_open and
> > qemu_close.
> You're not actually really removing that problem - this is just hacking
> around it in a manner which is racy & susceptible to silent failure.

Whatever happened to file-private locks (https://lwn.net/Articles/586904/)?
My very recent glibc doesn't appear to include them, unless the
final standard used something different from `F_SETLKPW'.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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