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sched: Kill the broken and deadlockable cpuset_lock/cpuset_cpus_allowed_locked code

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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>
This commit is contained in:
Oleg Nesterov
2010-03-15 10:10:03 +01:00
committed by Ingo Molnar
parent 25c2d55c00
commit 897f0b3c3f
3 changed files with 4 additions and 46 deletions
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27
kernel/cpuset.c
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@@ -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)
@@ -2382,22 +2373,6 @@ int __cpuset_node_allowed_hardwall(int node, gfp_t gfp_mask)
return 0;
}
/**
* 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
*