linux-kernel - [PATCH 4/3] x86/intel_rdt: Apply READ_ONCE/WRITE_ONCE to task

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Date:   Thu, 17 Dec 2020 12:19:31 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, bp@...en8.de,
        tony.luck@...el.com, kuo-lang.tseng@...el.com, shakeelb@...gle.com,
        mingo@...hat.com, babu.moger@....com, james.morse@....com,
        hpa@...or.com, Reinette Chatre <reinette.chatre@...el.com>
Subject: [PATCH 4/3] x86/intel_rdt: Apply READ_ONCE/WRITE_ONCE to task_struct .rmid & .closid

A CPU's current task can have its {closid, rmid} fields read locally while
they are being concurrently written to from another CPU. This can happen
anytime __resctrl_sched_in() races with either __rdtgroup_move_task() or
rdt_move_group_tasks().

Prevent load / store tearing for those accesses by giving them the
READ_ONCE() / WRITE_ONCE() treatment.

Signed-off-by: Valentin Schneider <valentin.schneider@....com>
---
 arch/x86/include/asm/resctrl.h         | 11 +++++++----
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 10 +++++-----
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
index 07603064df8f..d60ed0668a59 100644
--- a/arch/x86/include/asm/resctrl.h
+++ b/arch/x86/include/asm/resctrl.h
@@ -56,19 +56,22 @@ static void __resctrl_sched_in(void)
 	struct resctrl_pqr_state *state = this_cpu_ptr(&pqr_state);
 	u32 closid = state->default_closid;
 	u32 rmid = state->default_rmid;
+	u32 tmp;
 
 	/*
 	 * If this task has a closid/rmid assigned, use it.
 	 * Else use the closid/rmid assigned to this cpu.
 	 */
 	if (static_branch_likely(&rdt_alloc_enable_key)) {
-		if (current->closid)
-			closid = current->closid;
+		tmp = READ_ONCE(current->closid);
+		if (tmp)
+			closid = tmp;
 	}
 
 	if (static_branch_likely(&rdt_mon_enable_key)) {
-		if (current->rmid)
-			rmid = current->rmid;
+		tmp = READ_ONCE(current->rmid);
+		if (tmp)
+			rmid = tmp;
 	}
 
 	if (closid != state->cur_closid || rmid != state->cur_rmid) {
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 523660a68d9a..791a7064edea 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -601,11 +601,11 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
 	 */
 
 	if (rdtgrp->type == RDTCTRL_GROUP) {
-		tsk->closid = rdtgrp->closid;
-		tsk->rmid = rdtgrp->mon.rmid;
+		WRITE_ONCE(tsk->closid, rdtgrp->closid);
+		WRITE_ONCE(tsk->rmid, rdtgrp->mon.rmid);
 	} else if (rdtgrp->type == RDTMON_GROUP) {
 		if (rdtgrp->mon.parent->closid == tsk->closid) {
-			tsk->rmid = rdtgrp->mon.rmid;
+			WRITE_ONCE(tsk->rmid, rdtgrp->mon.rmid);
 		} else {
 			rdt_last_cmd_puts("Can't move task to different control group\n");
 			return -EINVAL;
@@ -2345,8 +2345,8 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
 	for_each_process_thread(p, t) {
 		if (!from || is_closid_match(t, from) ||
 		    is_rmid_match(t, from)) {
-			t->closid = to->closid;
-			t->rmid = to->mon.rmid;
+			WRITE_ONCE(t->closid, to->closid);
+			WRITE_ONCE(t->rmid, to->mon.rmid);
 
 			/* If the task is on a CPU, set the CPU in the mask. */
 			set_task_cpumask(t, mask);
-- 
2.27.0