Re: [Qemu-devel] [Qemu-trivial] [PATCH] keep the PID file locked for the lifetime of the process

[Laszlo Ersek <lersek@redhat.com>]

On 01/27/12 07:36, Stefan Hajnoczi wrote:
> On Thu, Jan 26, 2012 at 10:36:41PM +0100, Laszlo Ersek wrote:
>> The lockf() call in qemu_create_pidfile() aims at ensuring mutual
>> exclusion. We shouldn't close the pidfile on success, because that drops
>> the lock as well [1]:
>>
>>      "File locks shall be released on first close by the locking process
>>      of any file descriptor for the file."
>>
>> Coverity may complain again about the leaked file descriptor; let's
>> worry about that later.
>>
>> [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/lockf.html
>>
>> Signed-off-by: Laszlo Ersek<lersek@redhat.com>
>> ---
>
> Isn't the normal way to do pidfiles O_CREAT | O_EXCL?

Yes, it is.

> It may not work
> on all NFS versions but putting the pidfile on NFS doesn't really make
> sense.
>
> Then we can drop the lockf(3) completely.

When you rely on O_EXCL to ensure mutual exclusion, and an abruptly 
terminated process leaves the lockfile lying around, then the user has 
to clean it up manually before starting the next instance (and double 
check if the pid file is in fact stale or not). I'm personally OK with 
that, but I reckoned the qemu code tried to avoid that intentionally. 
Record locks can't remain stale when the process dies.

Laszlo