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Re: [Qemu-devel] [PATCH v3 08/27] osdep: Add qemu_lock_fd and qemu_unloc

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] [PATCH v3 08/27] osdep: Add qemu_lock_fd and qemu_unlock_fd
Date: Wed, 4 May 2016 14:46:32 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

On Thu, Apr 28, 2016 at 08:57:27PM +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.

> +    ret = qemu_lock_do(fd, start, len, readonly ? F_RDLCK : F_WRLCK);
> +    if (ret == -1) {
> +        return -errno;
> +    }

It still appears this patch would break libguestfs's valid use case.

How about addressing what Dan wrote here:




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