[PATCH] resctrl: Do not open directory for writing

[Michal Privoznik]

On 7/9/20 6:30 PM, Andrea Bolognani wrote:
> On Thu, 2020-07-09 at 13:44 +0200, Martin Kletzander wrote:
>> On Thu, Jul 09, 2020 at 12:50:38PM +0200, Andrea Bolognani wrote:
>>> it seems to me that this change entirely flipped the semantics of
>>> the lock.
>>
>> Yep, you're right.  When you have a lock that has boolean as a parameter I think
>> the boolean should indicate whether the lock is writable, but maybe the proper
>> and most readable solution would be virFileFlockShareable() and
>> virFileFlockExclusive() to avoid any misconception in the future[2].  Given the
>> fact that there are no (and most probably won't be) any other users of flock(2)
>> and given the fact that resctrl is also pretty niche feature, I don't have any
>> preference.  Also I feel like after some time I'm a little bit rusty with C and
>> the libvirt codebase (and most importantly the reasoning and decisions) has
>> changed a bit, so I'll leave the decision on how to deal with that on someone
>> else.  I'm happy to provide the patches when clear decision is made.
> 
> Either
> 
>    virFileFlockExclusive(fd)
>    virFileFlockShared(fd)
> 
> or
> 
>    virFileFlock(fd, VIR_FILE_FLOCK_EXCLUSIVE)
>    virFileFlock(fd, VIR_FILE_FLOCK_SHARED)
> 
> would work. I like the latter better because it's closer to the
> original flock(), which it ultimately acts as a very thin wrapper
> around of. I'm actually unclear why we'd have the last argument in
> the first place: personally I'd just use
> 
>    virFileFlock(fd, VIR_FILE_FLOCK_UNLOCK)
> 
> and keep things as unambiguous as they can be.
> 
> 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).

1: https://www.redhat.com/archives/libvir-list/2020-July/msg00451.html

Michal