[PATCH 2/2] x86/intel_rdt: Plug task_work vs task_struct {rmid,closid} update race

Valentin Schneider
2020-11-18T18:01:50Z

Upon moving a task to a new control / monitor group, said task's {closid,
rmid} fields are updated *after* triggering the move_myself() task_work
callback. This can cause said callback to miss the update, e.g. if the
triggering thread got preempted before fiddling with task_struct, or if the
targeted task was already on its way to return to userspace.

Update the task_struct's {closid, rmid} tuple *before* invoking
task_work_add(). As they can happen concurrently, wrap {closid, rmid}
accesses with READ_ONCE() and WRITE_ONCE(). Highlight the required ordering
with a pair of comments.

Fixes: e02737d5b826 ("x86/intel_rdt: Add tasks files")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
arch/x86/include/asm/resctrl.h | 11 ++++--
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 54 +++++++++++++++-----------
2 files changed, 39 insertions(+), 26 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 b6b5b95df833..135a51529f70 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -524,11 +524,13 @@ static void move_myself(struct callback_head *head)
* If resource group was deleted before this task work callback
* was invoked, then assign the task to root group and free the
* resource group.
+ *
+ * See pairing atomic_inc() in __rdtgroup_move_task()
*/
if (atomic_dec_and_test(&rdtgrp->waitcount) &&
(rdtgrp->flags & RDT_DELETED)) {
- current->closid = 0;
- current->rmid = 0;
+ WRITE_ONCE(current->closid, 0);
+ WRITE_ONCE(current->rmid, 0);
kfree(rdtgrp);
}

@@ -553,14 +555,32 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
callback = kzalloc(sizeof(*callback), GFP_KERNEL);
if (!callback)
return -ENOMEM;
- callback->work.func = move_myself;
+
+ init_task_work(&callback->work, move_myself);
callback->rdtgrp = rdtgrp;

+ /*
+ * For ctrl_mon groups move both closid and rmid.
+ * For monitor groups, can move the tasks only from
+ * their parent CTRL group.
+ */
+ if (rdtgrp->type == RDTCTRL_GROUP)
+ WRITE_ONCE(tsk->closid, rdtgrp->closid);
+ WRITE_ONCE(tsk->rmid, rdtgrp->mon.rmid);
+
/*
* Take a refcount, so rdtgrp cannot be freed before the
* callback has been invoked.
+ *
+ * Also ensures above {closid, rmid} writes are observed by
+ * move_myself(), as it can run immediately after task_work_add().
+ * Otherwise old values may be loaded, and the move will only actually
+ * happen at the next context switch.
+ *
+ * Pairs with atomic_dec() in move_myself().
*/
atomic_inc(&rdtgrp->waitcount);
+
ret = task_work_add(tsk, &callback->work, TWA_RESUME);
if (ret) {
/*
@@ -571,18 +591,6 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
atomic_dec(&rdtgrp->waitcount);
kfree(callback);
rdt_last_cmd_puts("Task exited\n");
- } else {
- /*
- * For ctrl_mon groups move both closid and rmid.
- * For monitor groups, can move the tasks only from
- * their parent CTRL group.
- */
- if (rdtgrp->type == RDTCTRL_GROUP) {
- tsk->closid = rdtgrp->closid;
- tsk->rmid = rdtgrp->mon.rmid;
- } else if (rdtgrp->type == RDTMON_GROUP) {
- tsk->rmid = rdtgrp->mon.rmid;
- }
}
return ret;
}
@@ -590,13 +598,15 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
static bool is_closid_match(struct task_struct *t, struct rdtgroup *r)
{
return (rdt_alloc_capable &&
- (r->type == RDTCTRL_GROUP) && (t->closid == r->closid));
+ (r->type == RDTCTRL_GROUP) &&
+ (READ_ONCE(t->closid) == r->closid));
}

static bool is_rmid_match(struct task_struct *t, struct rdtgroup *r)
{
return (rdt_mon_capable &&
- (r->type == RDTMON_GROUP) && (t->rmid == r->mon.rmid));
+ (r->type == RDTMON_GROUP) &&
+ (READ_ONCE(t->rmid) == r->mon.rmid));
}

/**
@@ -672,7 +682,7 @@ static int rdtgroup_move_task(pid_t pid, struct rdtgroup *rdtgrp,
goto out;

if (rdtgrp->type == RDTMON_GROUP &&
- rdtgrp->mon.parent->closid != tsk->closid) {
+ rdtgrp->mon.parent->closid != READ_ONCE(tsk->closid)) {
rdt_last_cmd_puts("Can't move task to different control group\n");
ret = -EINVAL;
goto out;
@@ -802,7 +812,7 @@ int proc_resctrl_show(struct seq_file *s, struct pid_namespace *ns,
rdtg->mode != RDT_MODE_EXCLUSIVE)
continue;

- if (rdtg->closid != tsk->closid)
+ if (rdtg->closid != READ_ONCE(tsk->closid))
continue;

seq_printf(s, "res:%s%s\n", (rdtg == &rdtgroup_default) ? "/" : "",
@@ -810,7 +820,7 @@ int proc_resctrl_show(struct seq_file *s, struct pid_namespace *ns,
seq_puts(s, "mon:");
list_for_each_entry(crg, &rdtg->mon.crdtgrp_list,
mon.crdtgrp_list) {
- if (tsk->rmid != crg->mon.rmid)
+ if (READ_ONCE(tsk->rmid) != crg->mon.rmid)
continue;
seq_printf(s, "%s", crg->kn->name);
break;
@@ -2328,8 +2338,8 @@ 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);

#ifdef CONFIG_SMP
/*
--
2.27.0