Hi Valentin,
On 12/16/2020 9:41 AM, Valentin Schneider wrote:
On 14/12/20 18:41, Reinette Chatre wrote:
- return ret;
+
+ /*
+ * By now, the task's closid and rmid are set. If the task is current
+ * on a CPU, the PQR_ASSOC MSR needs to be updated to make the resource
+ * group go into effect. If the task is not current, the MSR will be
+ * updated when the task is scheduled in.
+ */
+ update_task_closid_rmid(tsk);
We need the above writes to be compile-ordered before the IPI is sent.
There *is* a preempt_disable() down in smp_call_function_single() that
gives us the required barrier(), can we deem that sufficient or would we
want one before update_task_closid_rmid() for the sake of clarity?
Apologies, it is not clear to me why the preempt_disable() would be
insufficient. If it is not then there may be a few other areas (where
resctrl calls smp_call_function_xxx()) that needs to be re-evaluated.
So that's part paranoia and part nonsense from my end - the contents of
smp_call() shouldn't matter here.
If we distill the code to:
tsk->closid = x;
if (task_curr(tsk))
smp_call(...);
It is somewhat far fetched, but AFAICT this can be compiled as:
if (task_curr(tsk))
tsk->closid = x;
smp_call(...);
else
tsk->closid = x;
IOW, there could be a sequence where the closid write is ordered *after*
the task_curr() read.
Could you please elaborate why it would be an issue is the closid write
is ordered after the task_curr() read? task_curr() does not depend on
the closid.
With
tsk->closid = x;
barrier();
if (task_curr(tsk))
smp_call(...);
that explicitely cannot happen.
Reinette
