Re: [PATCH 4/4] sched/cache: Introduce task_struct->sched_cache_grp

[Chen Yu]

On Thu, Sep 10, 2026 at 03:50:56PM -0700, Tim Chen wrote:
> On Thu, 2026-09-10 at 21:19 +0200, Peter Zijlstra wrote:
> > On Thu, Sep 10, 2026 at 10:46:12AM -0700, Tim Chen wrote:
> > > @@ -882,6 +882,20 @@ static int exec_mmap(struct linux_binprm *bprm)
> > > +
> > > +		old_grp = rcu_dereference_protected(tsk->sched_cache_grp, true);
> > > +
> > 
> > Also: rcu_dereference_protected(.c = true) is another offence, that's
> > just wrong.

Got it, will fix it.
If I understand correctly, the "c" should never pass an explicit const value?
The original idea was that, in exec_mmap(), the current running task is the only
writer to change tsk->sched_cache_grp, so the condition is true,

To fix it, it might be turned into the following for now:
rcu_dereference_protected(tsk->sched_cache_grp, tsk == current);
like deref_curr_numa_group().

In the future if task tagging is supported, the race condition of multiple
updates would require a pi_lock for this task, and it will become:
rcu_dereference_protected(tsk->sched_cache_grp, lockdep_is_held(&p->pi_lock));
like what we have done in the prctl based cache aware scheduling patch set:
https://lore.kernel.org/lkml/50fe2db1a62ea2376a87d0c14778b1ff456d11ec.1787955777.git.tim.c.chen@xxxxxxxxxxxxxxx/

thanks,
Chenyu