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

[Eric Blake]

On 01/19/2017 09:49 AM, Daniel P. Berrange wrote:
> 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:
>>

>> +/* 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

Thankfully, fcntl() locks can be taken beyond end-of-file, so I think
we're okay making an arbitrarily larger range of bytes reserved for each
process, even in the unlikely corner case of passing files smaller than
512 bytes.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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