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authorPeter Zijlstra <peterz@infradead.org>2010-09-16 17:50:31 +0200
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2010-10-07 10:41:07 -0700
commit6506cf6ce68d78a5470a8360c965dafe8e4b78e3 (patch)
tree0eefe27d016572f1def9cbc2ad042d42b8a5e74f /kernel/sched.c
parent4ee0a603926cad973e4d384f48c5e279a0fd4118 (diff)
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sched: fix RCU lockdep splat from task_group()
This addresses the following RCU lockdep splat: [0.051203] CPU0: AMD QEMU Virtual CPU version 0.12.4 stepping 03 [0.052999] lockdep: fixing up alternatives. [0.054105] [0.054106] =================================================== [0.054999] [ INFO: suspicious rcu_dereference_check() usage. ] [0.054999] --------------------------------------------------- [0.054999] kernel/sched.c:616 invoked rcu_dereference_check() without protection! [0.054999] [0.054999] other info that might help us debug this: [0.054999] [0.054999] [0.054999] rcu_scheduler_active = 1, debug_locks = 1 [0.054999] 3 locks held by swapper/1: [0.054999] #0: (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff814be933>] cpu_up+0x42/0x6a [0.054999] #1: (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff810400d8>] cpu_hotplug_begin+0x2a/0x51 [0.054999] #2: (&rq->lock){-.-...}, at: [<ffffffff814be2f7>] init_idle+0x2f/0x113 [0.054999] [0.054999] stack backtrace: [0.054999] Pid: 1, comm: swapper Not tainted 2.6.35 #1 [0.054999] Call Trace: [0.054999] [<ffffffff81068054>] lockdep_rcu_dereference+0x9b/0xa3 [0.054999] [<ffffffff810325c3>] task_group+0x7b/0x8a [0.054999] [<ffffffff810325e5>] set_task_rq+0x13/0x40 [0.054999] [<ffffffff814be39a>] init_idle+0xd2/0x113 [0.054999] [<ffffffff814be78a>] fork_idle+0xb8/0xc7 [0.054999] [<ffffffff81068717>] ? mark_held_locks+0x4d/0x6b [0.054999] [<ffffffff814bcebd>] do_fork_idle+0x17/0x2b [0.054999] [<ffffffff814bc89b>] native_cpu_up+0x1c1/0x724 [0.054999] [<ffffffff814bcea6>] ? do_fork_idle+0x0/0x2b [0.054999] [<ffffffff814be876>] _cpu_up+0xac/0x127 [0.054999] [<ffffffff814be946>] cpu_up+0x55/0x6a [0.054999] [<ffffffff81ab562a>] kernel_init+0xe1/0x1ff [0.054999] [<ffffffff81003854>] kernel_thread_helper+0x4/0x10 [0.054999] [<ffffffff814c353c>] ? restore_args+0x0/0x30 [0.054999] [<ffffffff81ab5549>] ? kernel_init+0x0/0x1ff [0.054999] [<ffffffff81003850>] ? kernel_thread_helper+0x0/0x10 [0.056074] Booting Node 0, Processors #1lockdep: fixing up alternatives. [0.130045] #2lockdep: fixing up alternatives. [0.203089] #3 Ok. [0.275286] Brought up 4 CPUs [0.276005] Total of 4 processors activated (16017.17 BogoMIPS). The cgroup_subsys_state structures referenced by idle tasks are never freed, because the idle tasks should be part of the root cgroup, which is not removable. The problem is that while we do in-fact hold rq->lock, the newly spawned idle thread's cpu is not yet set to the correct cpu so the lockdep check in task_group(): lockdep_is_held(&task_rq(p)->lock) will fail. But this is a chicken and egg problem. Setting the CPU's runqueue requires that the CPU's runqueue already be set. ;-) So insert an RCU read-side critical section to avoid the complaint. Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/sched.c')
-rw-r--r--kernel/sched.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/kernel/sched.c b/kernel/sched.c
index dc85ceb9083..ae8f75a5ceb 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5337,7 +5337,19 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
idle->se.exec_start = sched_clock();
cpumask_copy(&idle->cpus_allowed, cpumask_of(cpu));
+ /*
+ * We're having a chicken and egg problem, even though we are
+ * holding rq->lock, the cpu isn't yet set to this cpu so the
+ * lockdep check in task_group() will fail.
+ *
+ * Similar case to sched_fork(). / Alternatively we could
+ * use task_rq_lock() here and obtain the other rq->lock.
+ *
+ * Silence PROVE_RCU
+ */
+ rcu_read_lock();
__set_task_cpu(idle, cpu);
+ rcu_read_unlock();
rq->curr = rq->idle = idle;
#if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)