[34-longterm 082/260] sched: Kill the broken and deadlockable cpuset_lock/cpuset_cpus_allowed_locked code

Paul Gortmaker
2011-01-02T07:22:56Z

From: Oleg Nesterov <oleg@redhat.com>

commit 897f0b3c3ff40b443c84e271bef19bd6ae885195 upstream.

This patch just states the fact the cpusets/cpuhotplug interaction is
broken and removes the deadlockable code which only pretends to work.

- cpuset_lock() doesn't really work. It is needed for
cpuset_cpus_allowed_locked() but we can't take this lock in
try_to_wake_up()->select_fallback_rq() path.

- cpuset_lock() is deadlockable. Suppose that a task T bound to CPU takes
callback_mutex. If cpu_down(CPU) happens before T drops callback_mutex
stop_machine() preempts T, then migration_call(CPU_DEAD) tries to take
cpuset_lock() and hangs forever because CPU is already dead and thus
T can't be scheduled.

- cpuset_cpus_allowed_locked() is deadlockable too. It takes task_lock()
which is not irq-safe, but try_to_wake_up() can be called from irq.

Kill them, and change select_fallback_rq() to use cpu_possible_mask, like
we currently do without CONFIG_CPUSETS.

Also, with or without this patch, with or without CONFIG_CPUSETS, the
callers of select_fallback_rq() can race with each other or with
set_cpus_allowed() pathes.

The subsequent patches try to to fix these problems.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100315091003.GA9123@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
include/linux/cpuset.h | 13 -------------
kernel/cpuset.c | 27 +--------------------------
kernel/sched.c | 10 +++-------
3 files changed, 4 insertions(+), 46 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index a5740fc..eeaaee7 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -21,8 +21,6 @@ extern int number_of_cpusets; /* How many cpusets are defined in system? */
extern int cpuset_init(void);
extern void cpuset_init_smp(void);
extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
-extern void cpuset_cpus_allowed_locked(struct task_struct *p,
- struct cpumask *mask);
extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
#define cpuset_current_mems_allowed (current->mems_allowed)
void cpuset_init_current_mems_allowed(void);
@@ -69,9 +67,6 @@ struct seq_file;
extern void cpuset_task_status_allowed(struct seq_file *m,
struct task_struct *task);

-extern void cpuset_lock(void);
-extern void cpuset_unlock(void);
-
extern int cpuset_mem_spread_node(void);

static inline int cpuset_do_page_mem_spread(void)
@@ -105,11 +100,6 @@ static inline void cpuset_cpus_allowed(struct task_struct *p,
{
cpumask_copy(mask, cpu_possible_mask);
}
-static inline void cpuset_cpus_allowed_locked(struct task_struct *p,
- struct cpumask *mask)
-{
- cpumask_copy(mask, cpu_possible_mask);
-}

static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
{
@@ -157,9 +147,6 @@ static inline void cpuset_task_status_allowed(struct seq_file *m,
{
}

-static inline void cpuset_lock(void) {}
-static inline void cpuset_unlock(void) {}
-
static inline int cpuset_mem_spread_node(void)
{
return 0;
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index d109467..9a747f5 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -2182,19 +2182,10 @@ void __init cpuset_init_smp(void)
void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
{
mutex_lock(&callback_mutex);
- cpuset_cpus_allowed_locked(tsk, pmask);
- mutex_unlock(&callback_mutex);
-}
-
-/**
- * cpuset_cpus_allowed_locked - return cpus_allowed mask from a tasks cpuset.
- * Must be called with callback_mutex held.
- **/
-void cpuset_cpus_allowed_locked(struct task_struct *tsk, struct cpumask *pmask)
-{
task_lock(tsk);
guarantee_online_cpus(task_cs(tsk), pmask);
task_unlock(tsk);
+ mutex_unlock(&callback_mutex);
}

void cpuset_init_current_mems_allowed(void)
@@ -2383,22 +2374,6 @@ int __cpuset_node_allowed_hardwall(int node, gfp_t gfp_mask)
}

/**
- * cpuset_lock - lock out any changes to cpuset structures
- *
- * The out of memory (oom) code needs to mutex_lock cpusets
- * from being changed while it scans the tasklist looking for a
- * task in an overlapping cpuset. Expose callback_mutex via this
- * cpuset_lock() routine, so the oom code can lock it, before
- * locking the task list. The tasklist_lock is a spinlock, so
- * must be taken inside callback_mutex.
- */
-
-void cpuset_lock(void)
-{
- mutex_lock(&callback_mutex);
-}
-
-/**
* cpuset_unlock - release lock on cpuset changes
*
* Undo the lock taken in a previous cpuset_lock() call.
diff --git a/kernel/sched.c b/kernel/sched.c
index a4a3801..489d985 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2302,11 +2302,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
return dest_cpu;

/* No more Mr. Nice Guy. */
- if (dest_cpu >= nr_cpu_ids) {
- rcu_read_lock();
- cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
- rcu_read_unlock();
- dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
+ if (unlikely(dest_cpu >= nr_cpu_ids)) {
+ cpumask_copy(&p->cpus_allowed, cpu_possible_mask);
+ dest_cpu = cpumask_any(cpu_active_mask);

/*
* Don't tell them about moving exiting tasks or
@@ -5947,7 +5945,6 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)

case CPU_DEAD:
case CPU_DEAD_FROZEN:
- cpuset_lock(); /* around calls to cpuset_cpus_allowed_lock() */
migrate_live_tasks(cpu);
rq = cpu_rq(cpu);
kthread_stop(rq->migration_thread);
@@ -5961,7 +5958,6 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
rq->idle->sched_class = &idle_sched_class;
migrate_dead_tasks(cpu);
raw_spin_unlock_irq(&rq->lock);
- cpuset_unlock();
migrate_nr_uninterruptible(rq);
BUG_ON(rq->nr_running != 0);
calc_global_load_remove(rq);
--
1.7.3.3