x86/resctrl: Fix task CLOSID/RMID update race

[ Upstream commit fe1f0714385fbcf76b0cbceb02b7277d842014fc ]

When the user moves a running task to a new rdtgroup using the task's
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

  [ bp: Massage commit message. ]

Fixes: 5a0f3e48f0d5 ("x86/resctrl: Use an IPI instead of task_work_add() to update PQR_ASSOC MSR")
Fixes: b6975e4cc679 ("x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount")
Signed-off-by: Peter Newman <peternewman@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20221220161123.432120-1-peternewman@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 2c19f2ecfa0326103f9c5eae9f13b5d8178368a9..8d6023e6ad9e389ae261438b739f8517094c0372 100644 (file)
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -577,8 +577,10 @@ 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
@@ -2178,6 +2180,14 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
                        t->closid = to->closid;
                        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