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sched: fix __set_task_cpu() SMP race 

Grant Wilson has reported rare SCHED_FAIR_USER crashes on his quad-core
system, which crashes can only be explained via runqueue corruption.

there is a narrow SMP race in __set_task_cpu(): after ->cpu is set up to
a new value, task_rq_lock(p, ...) can be successfuly executed on another
CPU. We must ensure that updates of per-task data have been completed by
this moment.

this bug has been hiding in the Linux scheduler for an eternity (we never
had any explicit barrier for task->cpu in set_task_cpu() - so the bug was
introduced in 2.5.1), but only became visible via set_task_cfs_rq() being
accidentally put after the task->cpu update. It also probably needs a
sufficiently out-of-order CPU to trigger.

Reported-by: Grant Wilson <grant.wilson@zen.co.uk>
Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Dmitry Adamushko 2007-11-15 20:57:40 +01:00 committed by Ingo Molnar
parent dae51f5620
commit ce96b5ac74
1 changed files with 13 additions and 7 deletions
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20
kernel/sched.c
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@ -216,15 +216,15 @@ static inline struct task_group *task_group(struct task_struct *p)
}
/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
static inline void set_task_cfs_rq(struct task_struct *p)
static inline void set_task_cfs_rq(struct task_struct *p, unsigned int cpu)
{
p->se.cfs_rq = task_group(p)->cfs_rq[task_cpu(p)];
p->se.parent = task_group(p)->se[task_cpu(p)];
p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
p->se.parent = task_group(p)->se[cpu];
}
#else
static inline void set_task_cfs_rq(struct task_struct *p) { }
static inline void set_task_cfs_rq(struct task_struct *p, unsigned int cpu) { }
#endif /* CONFIG_FAIR_GROUP_SCHED */
@ -1022,10 +1022,16 @@ unsigned long weighted_cpuload(const int cpu)
static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
{
set_task_cfs_rq(p, cpu);
#ifdef CONFIG_SMP
/*
* After ->cpu is set up to a new value, task_rq_lock(p, ...) can be
* successfuly executed on another CPU. We must ensure that updates of
* per-task data have been completed by this moment.
*/
smp_wmb();
task_thread_info(p)->cpu = cpu;
#endif
set_task_cfs_rq(p);
}
#ifdef CONFIG_SMP
@ -7088,7 +7094,7 @@ void sched_move_task(struct task_struct *tsk)
rq = task_rq_lock(tsk, &flags);
if (tsk->sched_class != &fair_sched_class) {
set_task_cfs_rq(tsk);
set_task_cfs_rq(tsk, task_cpu(tsk));
goto done;
}
@ -7103,7 +7109,7 @@ void sched_move_task(struct task_struct *tsk)
tsk->sched_class->put_prev_task(rq, tsk);
}
set_task_cfs_rq(tsk);
set_task_cfs_rq(tsk, task_cpu(tsk));
if (on_rq) {
if (unlikely(running))