Re: [PATCH] x86/mem_sharing: don't lock parent during fork reset

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Fri, Sep 17, 2021 at 3:26 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 16.09.2021 17:04, Tamas K Lengyel wrote:
> > During fork reset operation the parent domain doesn't need to be gathered 
> > using
> > rcu_lock_live_remote_domain_by_id as the fork reset doesn't modify anything 
> > on
> > the parent. The parent is also guaranteed to be paused while forks are 
> > active.
> > This patch reduces lock contention when performing resets in parallel.
>
> I'm a little in trouble following you here: RCU locks aren't really
> locks in that sense, so "lock contention" seems misleading to me. I
> can see that rcu_lock_domain_by_id()'s loop is extra overhead.
>
> Furthermore - does the parent being paused really mean the parent
> can't go away behind the back of the fork reset? In fork() I see
>
>     if ( rc && rc != -ERESTART )
>     {
>         domain_unpause(d);
>         put_domain(d);
>         cd->parent = NULL;
>     }
>
> i.e. the ref gets dropped before the parent pointer gets cleared. If
> the parent having a reference kept was indeed properly guaranteed, I
> agree the code change itself is fine.
>
> (The sequence looks correct at the other put_domain() site [dealing
> with the success case of fork(), when the reference gets retained]
> in domain_relinquish_resources().)

This code above you copied is when the fork() fails. Calling
fork_reset() before fork() successfully returns is not a sane sequence
and it is not "supported" by any means. If someone would try to do
that it would be racy as-is already with or without this patch.
Clearing the cd->parent pointer first here on the error path wouldn't
guarantee that sequence to be safe or sane either. Adding an extra
field to struct domain that signifies that "fork is complete" would be
a way to make that safe. But since the toolstack using this interface
is already sane (ie. never calls fork_reset before a successful fork)
I really don't think that's necessary. It would just grow struct
domain for very little benefit.

Tamas