[PATCH v5 1/1] x86/resctrl: Fix task CLOSID/RMID update race

[Peter Newman <peternewman@google.com>]

When the user moves a running task to a new rdtgroup using the tasks
file interface or by deleting its rdtgroup, the resulting change in
CLOSID/RMID must be immediately propagated to the PQR_ASSOC MSR on the
task(s) CPUs.

x86 allows reordering loads with prior stores, so if the task starts
running between a task_curr() check that the CPU hoisted before the
stores in the CLOSID/RMID update then it can start running with the old
CLOSID/RMID until it is switched again because __rdtgroup_move_task()
failed to determine that it needs to be interrupted to obtain the new
CLOSID/RMID.

Refer to the diagram below:

CPU 0                                   CPU 1
-----                                   -----
__rdtgroup_move_task():
  curr <- t1->cpu->rq->curr
                                        __schedule():
                                          rq->curr <- t1
                                        resctrl_sched_in():
                                          t1->{closid,rmid} -> {1,1}
  t1->{closid,rmid} <- {2,2}
  if (curr == t1) // false
   IPI(t1->cpu)

A similar race impacts rdt_move_group_tasks(), which updates tasks in a
deleted rdtgroup.

In both cases, use smp_mb() to order the task_struct::{closid,rmid}
stores before the loads in task_curr().  In particular, in the
rdt_move_group_tasks() case, simply execute an smp_mb() on every
iteration with a matching task.

It is possible to use a single smp_mb() in rdt_move_group_tasks(), but
this would require two passes and a means of remembering which
task_structs were updated in the first loop. However, benchmarking
results below showed too little performance impact in the simple
approach to justify implementing the two-pass approach.

Times below were collected using `perf stat` to measure the time to
remove a group containing a 1600-task, parallel workload.

CPU: Intel(R) Xeon(R) Platinum P-8136 CPU @ 2.00GHz (112 threads)

 # mkdir /sys/fs/resctrl/test
 # echo $$ > /sys/fs/resctrl/test/tasks
 # perf bench sched messaging -g 40 -l 100000

task-clock time ranges collected using:

 # perf stat rmdir /sys/fs/resctrl/test

Baseline:                     1.54 - 1.60 ms
smp_mb() every matching task: 1.57 - 1.67 ms

Signed-off-by: Peter Newman <peternewman@google.com>
---
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index e5a48f05e787..5993da21d822 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -580,8 +580,10 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
 	/*
 	 * Ensure the task's closid and rmid are written before determining if
 	 * the task is current that will decide if it will be interrupted.
+	 * This pairs with the full barrier between the rq->curr update and
+	 * resctrl_sched_in() during context switch.
 	 */
-	barrier();
+	smp_mb();
 
 	/*
 	 * By now, the task's closid and rmid are set. If the task is current
@@ -2401,6 +2403,14 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
 			WRITE_ONCE(t->closid, to->closid);
 			WRITE_ONCE(t->rmid, to->mon.rmid);
 
+			/*
+			 * Order the closid/rmid stores above before the loads
+			 * in task_curr(). This pairs with the full barrier
+			 * between the rq->curr update and resctrl_sched_in()
+			 * during context switch.
+			 */
+			smp_mb();
+
 			/*
 			 * If the task is on a CPU, set the CPU in the mask.
 			 * The detection is inaccurate as tasks might move or
-- 
2.39.0.rc1.256.g54fd8350bd-goog