Re: [Qemu-devel] [PATCH v10 14/16] file-posix: Implement image locking

[Daniel P. Berrange]

On Thu, Jan 19, 2017 at 10:38:14PM +0800, Fam Zheng wrote:
> This implements open flag sensible image locking for local file
> and host device protocol.
> 
> virtlockd in libvirt locks the first byte, so we start looking at the
> file bytes from 1.
> 
> Quoting what was proposed by Kevin Wolf <address@hidden>, there are
> four locking modes by combining two bits (BDRV_O_RDWR and
> BDRV_O_SHARE_RW), and implemented by taking two locks:
> 
> Lock bytes:
> 
> * byte 1: I can't allow other processes to write to the image
> * byte 2: I am writing to the image
> 
> Lock modes:
> 
> * shared writer (BDRV_O_RDWR | BDRV_O_SHARE_RW): Take shared lock on
>   byte 2. Test whether byte 1 is locked using an exclusive lock, and
>   fail if so.
> 
> * exclusive writer (BDRV_O_RDWR only): Take shared lock on byte 2. Test
>   whether byte 1 is locked using an exclusive lock, and fail if so. Then
>   take shared lock on byte 1. I suppose this is racy, but we can
>   probably tolerate that.
> 
> * reader that can tolerate writers (BDRV_O_SHARE_RW only): Don't do anything
> 
> * reader that can't tolerate writers (neither bit is set): Take shared
>   lock on byte 1. Test whether byte 2 is locked, and fail if so.

Ahh, using two bytes is an interesting technique for mapping the four
different access methods onto the more limit fcntl lock semantics. We
might want to copy this approach in libvirt too....

> +/* Posix file locking bytes. Libvirt takes byte 0, so start from byte 1. */
> +#define RAW_LOCK_BYTE_MIN             1
> +#define RAW_LOCK_BYTE_NO_OTHER_WRITER 1
> +#define RAW_LOCK_BYTE_WRITE           2

...would you mind if QEMU started from say byte 10, leaving the first 10
reserved for libvirt uses. This lets libvirt have a continuous space for
its own usage if we want to use more bytes

Regards,
Daniel
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