[PATCH v10 4/5] sched: Handle set_cpus_allowed_ptr(), sched_setaffinity() & other races

From: Waiman Long
Date: Thu Sep 22 2022 - 14:02:18 EST

Racing is possible between set_cpus_allowed_ptr() and sched_setaffinity()
or between multiple sched_setaffinity() calls from different
CPUs. To resolve these race conditions, we need to update both
user_cpus_ptr and cpus_mask in a single lock critical section instead
of separated ones. This requires moving the user_cpus_ptr update
to set_cpus_allowed_common() by putting the user_mask into a new
affinity_context structure and using it to pass information around
various functions.

This patch also changes the handling of the race between the
sched_setaffinity() call and the changing of cpumask of the current
cpuset. In case the new mask conflicts with newly updated cpuset,
the cpus_mask will be reset to the cpuset cpumask and an error value
of -EINVAL will be returned. If a previous user_cpus_ptr value exists,
it will be swapped back in and the new_mask will be further restricted
to what is allowed in the cpumask pointed to by the old user_cpus_ptr.

The potential race between sched_setaffinity() and a fork/clone()
syscall calling dup_user_cpus_ptr() is also being handled.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/sched/core.c | 169 ++++++++++++++++++++++++++--------------
kernel/sched/deadline.c | 7 +-
kernel/sched/sched.h | 12 ++-
3 files changed, 122 insertions(+), 66 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ab8e591dbaf5..ce626cad4105 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2195,14 +2195,18 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
#ifdef CONFIG_SMP

static void
-__do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32 flags);
+__do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx);

static int __set_cpus_allowed_ptr(struct task_struct *p,
- const struct cpumask *new_mask,
- u32 flags);
+ struct affinity_context *ctx);

static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
{
+ struct affinity_context ac = {
+ .new_mask = cpumask_of(rq->cpu),
+ .flags = SCA_MIGRATE_DISABLE,
+ };
+
if (likely(!p->migration_disabled))
return;

@@ -2212,7 +2216,7 @@ static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
/*
* Violates locking rules! see comment in __do_set_cpus_allowed().
*/
- __do_set_cpus_allowed(p, cpumask_of(rq->cpu), SCA_MIGRATE_DISABLE);
+ __do_set_cpus_allowed(p, &ac);
}

void migrate_disable(void)
@@ -2234,6 +2238,10 @@ EXPORT_SYMBOL_GPL(migrate_disable);
void migrate_enable(void)
{
struct task_struct *p = current;
+ struct affinity_context ac = {
+ .new_mask = &p->cpus_mask,
+ .flags = SCA_MIGRATE_ENABLE,
+ };

if (p->migration_disabled > 1) {
p->migration_disabled--;
@@ -2249,7 +2257,7 @@ void migrate_enable(void)
*/
preempt_disable();
if (p->cpus_ptr != &p->cpus_mask)
- __set_cpus_allowed_ptr(p, &p->cpus_mask, SCA_MIGRATE_ENABLE);
+ __set_cpus_allowed_ptr(p, &ac);
/*
* Mustn't clear migration_disabled() until cpus_ptr points back at the
* regular cpus_mask, otherwise things that race (eg.
@@ -2529,19 +2537,25 @@ int push_cpu_stop(void *arg)
* sched_class::set_cpus_allowed must do the below, but is not required to
* actually call this function.
*/
-void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_mask, u32 flags)
+void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx)
{
- if (flags & (SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) {
- p->cpus_ptr = new_mask;
+ if (ctx->flags & (SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) {
+ p->cpus_ptr = ctx->new_mask;
return;
}

- cpumask_copy(&p->cpus_mask, new_mask);
- p->nr_cpus_allowed = cpumask_weight(new_mask);
+ cpumask_copy(&p->cpus_mask, ctx->new_mask);
+ p->nr_cpus_allowed = cpumask_weight(ctx->new_mask);
+
+ /*
+ * Swap in a new user_cpus_ptr if SCA_USER flag set
+ */
+ if (ctx->flags & SCA_USER)
+ swap(p->user_cpus_ptr, ctx->user_mask);
}

static void
-__do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32 flags)
+__do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
{
struct rq *rq = task_rq(p);
bool queued, running;
@@ -2558,7 +2572,7 @@ __do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32
*
* XXX do further audits, this smells like something putrid.
*/
- if (flags & SCA_MIGRATE_DISABLE)
+ if (ctx->flags & SCA_MIGRATE_DISABLE)
SCHED_WARN_ON(!p->on_cpu);
else
lockdep_assert_held(&p->pi_lock);
@@ -2577,7 +2591,7 @@ __do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32
if (running)
put_prev_task(rq, p);

- p->sched_class->set_cpus_allowed(p, new_mask, flags);
+ p->sched_class->set_cpus_allowed(p, ctx);

if (queued)
enqueue_task(rq, p, ENQUEUE_RESTORE | ENQUEUE_NOCLOCK);
@@ -2587,12 +2601,19 @@ __do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32

void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
{
- __do_set_cpus_allowed(p, new_mask, 0);
+ struct affinity_context ac = {
+ .new_mask = new_mask,
+ .flags = 0,
+ };
+
+ __do_set_cpus_allowed(p, &ac);
}

int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
int node)
{
+ unsigned long flags;
+
if (!src->user_cpus_ptr)
return 0;

@@ -2600,7 +2621,10 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
if (!dst->user_cpus_ptr)
return -ENOMEM;

+ /* Use pi_lock to protect content of user_cpus_ptr */
+ raw_spin_lock_irqsave(&src->pi_lock, flags);
cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+ raw_spin_unlock_irqrestore(&src->pi_lock, flags);
return 0;
}

@@ -2840,8 +2864,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
* Called with both p->pi_lock and rq->lock held; drops both before returning.
*/
static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
- const struct cpumask *new_mask,
- u32 flags,
+ struct affinity_context *ctx,
struct rq *rq,
struct rq_flags *rf)
__releases(rq->lock)
@@ -2869,7 +2892,7 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
cpu_valid_mask = cpu_online_mask;
}

- if (!kthread && !cpumask_subset(new_mask, cpu_allowed_mask)) {
+ if (!kthread && !cpumask_subset(ctx->new_mask, cpu_allowed_mask)) {
ret = -EINVAL;
goto out;
}
@@ -2878,18 +2901,18 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
* Must re-check here, to close a race against __kthread_bind(),
* sched_setaffinity() is not guaranteed to observe the flag.
*/
- if ((flags & SCA_CHECK) && (p->flags & PF_NO_SETAFFINITY)) {
+ if ((ctx->flags & SCA_CHECK) && (p->flags & PF_NO_SETAFFINITY)) {
ret = -EINVAL;
goto out;
}

- if (!(flags & SCA_MIGRATE_ENABLE)) {
- if (cpumask_equal(&p->cpus_mask, new_mask))
+ if (!(ctx->flags & SCA_MIGRATE_ENABLE)) {
+ if (cpumask_equal(&p->cpus_mask, ctx->new_mask))
goto out;

if (WARN_ON_ONCE(p == current &&
is_migration_disabled(p) &&
- !cpumask_test_cpu(task_cpu(p), new_mask))) {
+ !cpumask_test_cpu(task_cpu(p), ctx->new_mask))) {
ret = -EBUSY;
goto out;
}
@@ -2900,15 +2923,15 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
* for groups of tasks (ie. cpuset), so that load balancing is not
* immediately required to distribute the tasks within their new mask.
*/
- dest_cpu = cpumask_any_and_distribute(cpu_valid_mask, new_mask);
+ dest_cpu = cpumask_any_and_distribute(cpu_valid_mask, ctx->new_mask);
if (dest_cpu >= nr_cpu_ids) {
ret = -EINVAL;
goto out;
}

- __do_set_cpus_allowed(p, new_mask, flags);
+ __do_set_cpus_allowed(p, ctx);

- return affine_move_task(rq, p, rf, dest_cpu, flags);
+ return affine_move_task(rq, p, rf, dest_cpu, ctx->flags);

out:
task_rq_unlock(rq, p, rf);
@@ -2926,7 +2949,7 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
* call is not atomic; no spinlocks may be held.
*/
static int __set_cpus_allowed_ptr(struct task_struct *p,
- const struct cpumask *new_mask, u32 flags)
+ struct affinity_context *ctx)
{
struct rq_flags rf;
struct rq *rq;
@@ -2937,16 +2960,21 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
* flags are set.
*/
if (p->user_cpus_ptr &&
- !(flags & (SCA_USER | SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) &&
- cpumask_and(rq->scratch_mask, new_mask, p->user_cpus_ptr))
- new_mask = rq->scratch_mask;
+ !(ctx->flags & (SCA_USER | SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) &&
+ cpumask_and(rq->scratch_mask, ctx->new_mask, p->user_cpus_ptr))
+ ctx->new_mask = rq->scratch_mask;

- return __set_cpus_allowed_ptr_locked(p, new_mask, flags, rq, &rf);
+ return __set_cpus_allowed_ptr_locked(p, ctx, rq, &rf);
}

int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
{
- return __set_cpus_allowed_ptr(p, new_mask, 0);
+ struct affinity_context ac = {
+ .new_mask = new_mask,
+ .flags = 0,
+ };
+
+ return __set_cpus_allowed_ptr(p, &ac);
}
EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);

@@ -2963,6 +2991,10 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
struct cpumask *new_mask,
const struct cpumask *subset_mask)
{
+ struct affinity_context ac = {
+ .new_mask = new_mask,
+ .flags = 0,
+ };
struct rq_flags rf;
struct rq *rq;
int err;
@@ -2984,7 +3016,7 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
goto err_unlock;
}

- return __set_cpus_allowed_ptr_locked(p, new_mask, 0, rq, &rf);
+ return __set_cpus_allowed_ptr_locked(p, &ac, rq, &rf);

err_unlock:
task_rq_unlock(rq, p, &rf);
@@ -3037,7 +3069,7 @@ void force_compatible_cpus_allowed_ptr(struct task_struct *p)
}

static int
-__sched_setaffinity(struct task_struct *p, const struct cpumask *mask, int flags);
+__sched_setaffinity(struct task_struct *p, struct affinity_context *ctx);

/*
* Restore the affinity of a task @p which was previously restricted by a
@@ -3048,13 +3080,17 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask, int flags
*/
void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
{
+ struct affinity_context ac = {
+ .new_mask = task_user_cpus(p),
+ .flags = 0,
+ };
int ret;

/*
* Try to restore the old affinity mask with __sched_setaffinity().
* Cpuset masking will be done there too.
*/
- ret = __sched_setaffinity(p, task_user_cpus(p), 0);
+ ret = __sched_setaffinity(p, &ac);
WARN_ON_ONCE(ret);
}

@@ -3533,10 +3569,9 @@ void sched_set_stop_task(int cpu, struct task_struct *stop)
#else /* CONFIG_SMP */

static inline int __set_cpus_allowed_ptr(struct task_struct *p,
- const struct cpumask *new_mask,
- u32 flags)
+ struct affinity_context *ctx)
{
- return set_cpus_allowed_ptr(p, new_mask);
+ return set_cpus_allowed_ptr(p, ctx->new_mask);
}

static inline void migrate_disable_switch(struct rq *rq, struct task_struct *p) { }
@@ -8058,7 +8093,7 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
#endif

static int
-__sched_setaffinity(struct task_struct *p, const struct cpumask *mask, int flags)
+__sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
{
int retval;
cpumask_var_t cpus_allowed, new_mask;
@@ -8072,13 +8107,16 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask, int flags
}

cpuset_cpus_allowed(p, cpus_allowed);
- cpumask_and(new_mask, mask, cpus_allowed);
+ cpumask_and(new_mask, ctx->new_mask, cpus_allowed);
+
+ ctx->new_mask = new_mask;
+ ctx->flags |= SCA_CHECK;

retval = dl_task_check_affinity(p, new_mask);
if (retval)
goto out_free_new_mask;
-again:
- retval = __set_cpus_allowed_ptr(p, new_mask, SCA_CHECK | flags);
+
+ retval = __set_cpus_allowed_ptr(p, ctx);
if (retval)
goto out_free_new_mask;

@@ -8089,7 +8127,24 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask, int flags
* Just reset the cpumask to the cpuset's cpus_allowed.
*/
cpumask_copy(new_mask, cpus_allowed);
- goto again;
+
+ /*
+ * If SCA_USER is set, a 2nd call to __set_cpus_allowed_ptr()
+ * will restore the previous user_cpus_ptr value.
+ *
+ * In the unlikely event a previous user_cpus_ptr exists,
+ * we need to further restrict the mask to what is allowed
+ * by that old user_cpus_ptr.
+ */
+ if (unlikely((ctx->flags & SCA_USER) && ctx->user_mask)) {
+ bool empty = !cpumask_and(new_mask, new_mask,
+ ctx->user_mask);
+
+ if (WARN_ON_ONCE(empty))
+ cpumask_copy(new_mask, cpus_allowed);
+ }
+ __set_cpus_allowed_ptr(p, ctx);
+ retval = -EINVAL;
}

out_free_new_mask:
@@ -8101,6 +8156,7 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask, int flags

long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
{
+ struct affinity_context ac;
struct cpumask *user_mask;
struct task_struct *p;
int retval;
@@ -8142,23 +8198,14 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
goto out_put_task;
}
cpumask_copy(user_mask, in_mask);
+ ac = (struct affinity_context){
+ .new_mask = in_mask,
+ .user_mask = user_mask,
+ .flags = SCA_USER,
+ };

- retval = __sched_setaffinity(p, in_mask, SCA_USER);
-
- /*
- * Save in_mask into user_cpus_ptr after a successful
- * __sched_setaffinity() call. pi_lock is used to synchronize
- * changes to user_cpus_ptr.
- */
- if (!retval) {
- unsigned long flags;
-
- /* Use pi_lock to synchronize changes to user_cpus_ptr */
- raw_spin_lock_irqsave(&p->pi_lock, flags);
- swap(p->user_cpus_ptr, user_mask);
- raw_spin_unlock_irqrestore(&p->pi_lock, flags);
- }
- kfree(user_mask);
+ retval = __sched_setaffinity(p, &ac);
+ kfree(ac.user_mask);

out_put_task:
put_task_struct(p);
@@ -8940,6 +8987,12 @@ void show_state_filter(unsigned int state_filter)
*/
void __init init_idle(struct task_struct *idle, int cpu)
{
+#ifdef CONFIG_SMP
+ struct affinity_context ac = (struct affinity_context) {
+ .new_mask = cpumask_of(cpu),
+ .flags = 0,
+ };
+#endif
struct rq *rq = cpu_rq(cpu);
unsigned long flags;

@@ -8964,7 +9017,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
*
* And since this is boot we can forgo the serialization.
*/
- set_cpus_allowed_common(idle, cpumask_of(cpu), 0);
+ set_cpus_allowed_common(idle, &ac);
#endif
/*
* We're having a chicken and egg problem, even though we are
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0ab79d819a0d..38fa2c3ef7db 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2486,8 +2486,7 @@ static void task_woken_dl(struct rq *rq, struct task_struct *p)
}

static void set_cpus_allowed_dl(struct task_struct *p,
- const struct cpumask *new_mask,
- u32 flags)
+ struct affinity_context *ctx)
{
struct root_domain *src_rd;
struct rq *rq;
@@ -2502,7 +2501,7 @@ static void set_cpus_allowed_dl(struct task_struct *p,
* update. We already made space for us in the destination
* domain (see cpuset_can_attach()).
*/
- if (!cpumask_intersects(src_rd->span, new_mask)) {
+ if (!cpumask_intersects(src_rd->span, ctx->new_mask)) {
struct dl_bw *src_dl_b;

src_dl_b = dl_bw_of(cpu_of(rq));
@@ -2516,7 +2515,7 @@ static void set_cpus_allowed_dl(struct task_struct *p,
raw_spin_unlock(&src_dl_b->lock);
}

- set_cpus_allowed_common(p, new_mask, flags);
+ set_cpus_allowed_common(p, ctx);
}

/* Assumes rq->lock is held */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 482b702d65ea..110e13b7d78b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2157,6 +2157,12 @@ extern const u32 sched_prio_to_wmult[40];

#define RETRY_TASK ((void *)-1UL)

+struct affinity_context {
+ const struct cpumask *new_mask;
+ struct cpumask *user_mask;
+ unsigned int flags;
+};
+
struct sched_class {

#ifdef CONFIG_UCLAMP_TASK
@@ -2185,9 +2191,7 @@ struct sched_class {

void (*task_woken)(struct rq *this_rq, struct task_struct *task);

- void (*set_cpus_allowed)(struct task_struct *p,
- const struct cpumask *newmask,
- u32 flags);
+ void (*set_cpus_allowed)(struct task_struct *p, struct affinity_context *ctx);

void (*rq_online)(struct rq *rq);
void (*rq_offline)(struct rq *rq);
@@ -2301,7 +2305,7 @@ extern void update_group_capacity(struct sched_domain *sd, int cpu);

extern void trigger_load_balance(struct rq *rq);

-extern void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_mask, u32 flags);
+extern void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx);

static inline struct task_struct *get_push_task(struct rq *rq)
{
--
2.31.1