[PATCH] resctrl: Do not open directory for writing

[Michal Privoznik]

On 7/10/20 3:39 PM, Andrea Bolognani wrote:
> On Fri, 2020-07-10 at 15:13 +0200, Martin Kletzander wrote:
>> On Fri, Jul 10, 2020 at 10:47:22AM +0200, Michal Privoznik wrote:
>>> On 7/9/20 6:30 PM, Andrea Bolognani wrote:
>>>> This is all bikeshedding, of course: what actually matters is making
>>>> that lock exclusive once again :)
>>>
>>> Just realized that for exclusive (aka write) lock, the FD must be opened
>>> for writing (working on patches for the following report [1] and been
>>> experimenting a bit and that's what I'm seeing).
>>
>> Good point, but luckily not related to flock(2).
> 
> That seems to be the case: according to flock(2),
> 
>    A shared or exclusive lock can be placed on a file regardless
>    of the mode in which the file was opened.
> 
> Michal, does that sound reasonable to you?
> 

D'oh! of course this is another case of file locking exemptions. The 
patches I sent earlier today fix code around virFileLock() which is 
fcntl() which is POSIX locking. flock(2) is BSD lock which may or may 
not be implementedusing POSIX locks.

So I think we're okay on that front.
Alternatively, we may switch to OFD (F_OFD_SETLK from fcntl(2)) and 
experience proper file locking. Those are Linux only (but so is resctrl).

Michal