Re: [Qemu-devel] [PATCH v15 19/21] osdep: Add qemu_lock_fd and qemu_unlock_fd

[Kevin Wolf <kwolf@redhat.com>]

Am 26.04.2017 um 05:34 hat Fam Zheng geschrieben:
> They are wrappers of POSIX fcntl "file private locking", with a
> convenient "try lock" wrapper implemented with F_OFD_GETLK.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>  include/qemu/osdep.h |  3 +++
>  util/osdep.c         | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+)
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 122ff06..1c9f5e2 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -341,6 +341,9 @@ int qemu_close(int fd);
>  #ifndef _WIN32
>  int qemu_dup(int fd);
>  #endif
> +int qemu_lock_fd(int fd, int64_t start, int64_t len, bool exclusive);
> +int qemu_unlock_fd(int fd, int64_t start, int64_t len);
> +int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive);

For the record: On IRC, I proposed adding something like the following:

    #ifndef F_OFD_SETLK
    #define F_OFD_SETLK F_SETLK
    #define F_OFD_GETLK F_GETLK
    #endif

F_OFD_* are still relatively new and e.g. RHEL 7 doesn't support it yet.
Using process-based locks is suboptimal because we can easily lose them
earlier than we want, but it's still better than nothing and covers the
common simple cases.

Kevin