Messages in this thread Patch in this message | | | Date | Thu, 8 Sep 2022 13:53:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v7 4/5] sched: Handle set_cpus_allowed_ptr() & sched_setaffinity() race |
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On Fri, Sep 02, 2022 at 11:25:55AM -0400, Waiman Long wrote: > 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(). > > The SCA_USER flag will be used to signify that a user_cpus_ptr update > will have to be done. The new user_cpus_ptr will be put into the > a percpu variable pending_user_mask at the beginning of the lock > crtical section. The pending user mask will then be taken up in > set_cpus_allowed_common(). > > Ideally, user_cpus_ptr should only be updated if the sched_setaffinity() > is successful. However, this patch will update user_cpus_ptr when the > first call to __set_cpus_allowed_ptr() is successful. However, if there > is racing between sched_setaffinity() and cpuset update, the subsequent > calls to __set_cpus_allowed_ptr() may fail but the user_cpus_ptr will > still be updated in this corner case. A warning will be printed in this > corner case. > > Signed-off-by: Waiman Long <longman@redhat.com>
OK, so obviously this is terrible :/
What's wrong with something like so ?
--- kernel/sched/core.c | 220 ++++++++++++++++++++++++++++++------------------ kernel/sched/deadline.c | 7 +- kernel/sched/sched.h | 22 ++++- 3 files changed, 157 insertions(+), 92 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ee28253c9ac0..01ee81f347b1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2195,14 +2195,19 @@ 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 = { + .user_mask = NULL, + .new_mask = cpumask_of(rq->cpu), + .flags = SCA_MIGRATE_DISABLE, + }; + if (likely(!p->migration_disabled)) return; @@ -2212,7 +2217,7 @@ 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 +2239,11 @@ EXPORT_SYMBOL_GPL(migrate_disable); void migrate_enable(void) { struct task_struct *p = current; + struct affinity_context ac = { + .user_mask = NULL, + .new_mask = &p->cpus_mask, + .flags = SCA_MIGRATE_ENABLE, + }; if (p->migration_disabled > 1) { p->migration_disabled--; @@ -2249,7 +2259,7 @@ 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 +2539,22 @@ 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); + if (ctx->flags & SCA_USER) + swap(p->user_cpus_ptr, ctx->user_mask); + + cpumask_copy(&p->cpus_mask, ctx->new_mask); + p->nr_cpus_allowed = cpumask_weight(ctx->new_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 +2571,7 @@ __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 +2590,7 @@ __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); @@ -2585,13 +2598,23 @@ __do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask, u32 set_next_task(rq, p); } +/* + * Used for kthread_bind() and select_fallback_rq(), in both cases the user + * affinity (if any) should be destroyed too. + */ 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 = { + .user_mask = NULL, + .new_mask = new_mask, + .flags = SCA_USER, /* clear the user requested mask */ + }; + + __do_set_cpus_allowed(p, &ac); + kfree(ac.user_mask); } -int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, - int node) +int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, int node) { if (!src->user_cpus_ptr) return 0; @@ -2696,6 +2719,8 @@ void release_user_cpus_ptr(struct task_struct *p) */ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flags *rf, int dest_cpu, unsigned int flags) + __releases(rq->lock) + __releases(p->pi_lock) { struct set_affinity_pending my_pending = { }, *pending = NULL; bool stop_pending, complete = false; @@ -2838,8 +2863,7 @@ 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) @@ -2848,7 +2872,6 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p, const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p); const struct cpumask *cpu_valid_mask = cpu_active_mask; bool kthread = p->flags & PF_KTHREAD; - struct cpumask *user_mask = NULL; unsigned int dest_cpu; int ret = 0; @@ -2868,7 +2891,7 @@ 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; } @@ -2877,18 +2900,18 @@ 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; } @@ -2899,22 +2922,15 @@ 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); - if (flags & SCA_USER) - user_mask = clear_user_cpus_ptr(p); - - ret = affine_move_task(rq, p, rf, dest_cpu, flags); - - kfree(user_mask); - - return ret; + return affine_move_task(rq, p, rf, dest_cpu, ctx->flags); out: task_rq_unlock(rq, p, rf); @@ -2932,25 +2948,46 @@ 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; rq = task_rq_lock(p, &rf); - return __set_cpus_allowed_ptr_locked(p, new_mask, flags, rq, &rf); + + /* + * When this is not a user requestd affinity change, ensure the + * affinity request respects the previous user affinity if at all + * possible. + * + * XXX why can't we allocated scratch_mask before task_rq_lock() + * and return -ENOMEM here? + */ + if (p->user_cpus_ptr && !(ctx->flags & SCA_USER) && + 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, 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 = { + .user_mask = NULL, + .new_mask = new_mask, + .flags = 0, + }; + + return __set_cpus_allowed_ptr(p, &ac); } EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr); /* * Change a given task's CPU affinity to the intersection of its current - * affinity mask and @subset_mask, writing the resulting mask to @new_mask - * and pointing @p->user_cpus_ptr to a copy of the old mask. + * affinity mask and @subset_mask, writing the resulting mask to @new_mask. + * If user_cpus_ptr is defined, use it as the basis for restricting CPU + * affinity or use cpu_online_mask instead. + * * If the resulting mask is empty, leave the affinity unchanged and return * -EINVAL. */ @@ -2958,17 +2995,15 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p, struct cpumask *new_mask, const struct cpumask *subset_mask) { - struct cpumask *user_mask = NULL; + struct affinity_context ac = { + .user_mask = NULL, + .new_mask = new_mask, + .flags = 0, + }; struct rq_flags rf; struct rq *rq; int err; - if (!p->user_cpus_ptr) { - user_mask = kmalloc(cpumask_size(), GFP_KERNEL); - if (!user_mask) - return -ENOMEM; - } - rq = task_rq_lock(p, &rf); /* @@ -2981,31 +3016,21 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p, goto err_unlock; } - if (!cpumask_and(new_mask, &p->cpus_mask, subset_mask)) { + if (!cpumask_and(new_mask, task_user_cpus(p), subset_mask)) { err = -EINVAL; goto err_unlock; } - /* - * We're about to butcher the task affinity, so keep track of what - * the user asked for in case we're able to restore it later on. - */ - if (user_mask) { - cpumask_copy(user_mask, p->cpus_ptr); - p->user_cpus_ptr = user_mask; - } - - 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); - kfree(user_mask); return err; } /* * Restrict the CPU affinity of task @p so that it is a subset of - * task_cpu_possible_mask() and point @p->user_cpu_ptr to a copy of the + * task_cpu_possible_mask() and point @p->user_cpus_ptr to a copy of the * old affinity mask. If the resulting mask is empty, we warn and walk * up the cpuset hierarchy until we find a suitable mask. */ @@ -3049,34 +3074,30 @@ void force_compatible_cpus_allowed_ptr(struct task_struct *p) } static int -__sched_setaffinity(struct task_struct *p, const struct cpumask *mask); +__sched_setaffinity(struct task_struct *p, struct affinity_context *ctx); /* * Restore the affinity of a task @p which was previously restricted by a - * call to force_compatible_cpus_allowed_ptr(). This will clear (and free) - * @p->user_cpus_ptr. + * call to force_compatible_cpus_allowed_ptr(). * * It is the caller's responsibility to serialise this with any calls to * force_compatible_cpus_allowed_ptr(@p). */ void relax_compatible_cpus_allowed_ptr(struct task_struct *p) { - struct cpumask *user_mask = p->user_cpus_ptr; - unsigned long flags; + struct affinity_context ac = { + .user_mask = NULL, + .new_mask = task_user_cpus(p), + .flags = 0, + }; + int ret; /* - * Try to restore the old affinity mask. If this fails, then - * we free the mask explicitly to avoid it being inherited across - * a subsequent fork(). + * Try to restore the old affinity mask with __sched_setaffinity(). + * Cpuset masking will be done there too. */ - if (!user_mask || !__sched_setaffinity(p, user_mask)) - return; - - raw_spin_lock_irqsave(&p->pi_lock, flags); - user_mask = clear_user_cpus_ptr(p); - raw_spin_unlock_irqrestore(&p->pi_lock, flags); - - kfree(user_mask); + ret = __sched_setaffinity(p, &ac); + WARN_ON_ONCE(ret); } void set_task_cpu(struct task_struct *p, unsigned int new_cpu) @@ -3554,10 +3575,9 @@ 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) { } @@ -8079,10 +8099,10 @@ 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) +__sched_setaffinity(struct task_struct *p, struct affinity_context *ctx) { - int retval; cpumask_var_t cpus_allowed, new_mask; + int retval; if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) return -ENOMEM; @@ -8093,13 +8113,16 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask) } 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 | SCA_USER); + retval = __set_cpus_allowed_ptr(p, ctx); if (retval) goto out_free_new_mask; @@ -8108,6 +8131,8 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask) /* * We must have raced with a concurrent cpuset update. * Just reset the cpumask to the cpuset's cpus_allowed. + * + * XXX can we do better here? */ cpumask_copy(new_mask, cpus_allowed); goto again; @@ -8122,6 +8147,8 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask) 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; @@ -8156,7 +8183,23 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask) if (retval) goto out_put_task; - retval = __sched_setaffinity(p, in_mask); + user_mask = kmalloc(cpumask_size(), GFP_KERNEL); + if (!user_mask) { + retval = -ENOMEM; + goto out_put_task; + } + cpumask_copy(user_mask, in_mask); + + ac = (struct affinity_context){ + .user_mask = user_mask, + .new_mask = in_mask, + .flags = SCA_USER, + }; + + retval = __sched_setaffinity(p, &ac); + + kfree(user_mask); + out_put_task: put_task_struct(p); return retval; @@ -8938,6 +8981,7 @@ void show_state_filter(unsigned int state_filter) void __init init_idle(struct task_struct *idle, int cpu) { struct rq *rq = cpu_rq(cpu); + struct affinity_context ac; unsigned long flags; __sched_fork(0, idle); @@ -8955,13 +8999,18 @@ void __init init_idle(struct task_struct *idle, int cpu) kthread_set_per_cpu(idle, cpu); #ifdef CONFIG_SMP + ac = (struct affinity_context) { + .user_mask = NULL, + .new_mask = cpumask_of(cpu), + .flags = 0, + }; /* * It's possible that init_idle() gets called multiple times on a task, * in that case do_set_cpus_allowed() will not do the right thing. * * 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 @@ -9653,6 +9702,9 @@ void __init sched_init(void) cpumask_size(), GFP_KERNEL, cpu_to_node(i)); per_cpu(select_rq_mask, i) = (cpumask_var_t)kzalloc_node( cpumask_size(), GFP_KERNEL, cpu_to_node(i)); + per_cpu(runqueues.scratch_mask, i) = + (cpumask_var_t)kzalloc_node(cpumask_size(), + GFP_KERNEL, cpu_to_node(i)); } #endif /* CONFIG_CPUMASK_OFFSTACK */ 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 e26688d387ae..d53682557db7 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1159,6 +1159,9 @@ struct rq { unsigned int core_forceidle_occupation; u64 core_forceidle_start; #endif + + /* Scratch cpumask to be temporarily used under rq_lock */ + cpumask_var_t scratch_mask; }; #ifdef CONFIG_FAIR_GROUP_SCHED @@ -1881,6 +1884,13 @@ static inline void dirty_sched_domain_sysctl(int cpu) #endif extern int sched_update_scaling(void); + +static inline const struct cpumask *task_user_cpus(struct task_struct *p) +{ + if (!p->user_cpus_ptr) + return cpu_possible_mask; /* &init_task.cpus_mask */ + return p->user_cpus_ptr; +} #endif /* CONFIG_SMP */ #include "stats.h" @@ -2147,6 +2157,12 @@ extern const u32 sched_prio_to_wmult[40]; #define RETRY_TASK ((void *)-1UL) +struct affinity_context { + struct cpumask *user_mask; + const struct cpumask *new_mask; + unsigned int flags; +}; + struct sched_class { #ifdef CONFIG_UCLAMP_TASK @@ -2175,9 +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); @@ -2291,7 +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) {
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