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![Paul E. McKenney](/assets/img/avatar_default.png)
If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
645 lines
17 KiB
C
645 lines
17 KiB
C
/*
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* Read-Copy Update mechanism for mutual exclusion (tree-based version)
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* Internal non-public definitions that provide either classic
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* or preemptable semantics.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright Red Hat, 2009
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* Copyright IBM Corporation, 2009
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*
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* Author: Ingo Molnar <mingo@elte.hu>
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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*/
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#ifdef CONFIG_TREE_PREEMPT_RCU
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struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt_state);
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DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);
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/*
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* Tell them what RCU they are running.
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*/
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static inline void rcu_bootup_announce(void)
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{
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printk(KERN_INFO
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"Experimental preemptable hierarchical RCU implementation.\n");
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}
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/*
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* Return the number of RCU-preempt batches processed thus far
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* for debug and statistics.
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*/
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long rcu_batches_completed_preempt(void)
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{
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return rcu_preempt_state.completed;
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}
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EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);
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/*
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* Return the number of RCU batches processed thus far for debug & stats.
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*/
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long rcu_batches_completed(void)
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{
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return rcu_batches_completed_preempt();
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}
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EXPORT_SYMBOL_GPL(rcu_batches_completed);
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/*
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* Record a preemptable-RCU quiescent state for the specified CPU. Note
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* that this just means that the task currently running on the CPU is
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* not in a quiescent state. There might be any number of tasks blocked
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* while in an RCU read-side critical section.
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*/
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static void rcu_preempt_qs(int cpu)
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{
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struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
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rdp->passed_quiesc_completed = rdp->completed;
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barrier();
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rdp->passed_quiesc = 1;
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}
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/*
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* We have entered the scheduler, and the current task might soon be
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* context-switched away from. If this task is in an RCU read-side
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* critical section, we will no longer be able to rely on the CPU to
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* record that fact, so we enqueue the task on the appropriate entry
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* of the blocked_tasks[] array. The task will dequeue itself when
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* it exits the outermost enclosing RCU read-side critical section.
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* Therefore, the current grace period cannot be permitted to complete
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* until the blocked_tasks[] entry indexed by the low-order bit of
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* rnp->gpnum empties.
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*
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* Caller must disable preemption.
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*/
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static void rcu_preempt_note_context_switch(int cpu)
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{
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struct task_struct *t = current;
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unsigned long flags;
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int phase;
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struct rcu_data *rdp;
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struct rcu_node *rnp;
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if (t->rcu_read_lock_nesting &&
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(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
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/* Possibly blocking in an RCU read-side critical section. */
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rdp = rcu_preempt_state.rda[cpu];
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rnp = rdp->mynode;
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spin_lock_irqsave(&rnp->lock, flags);
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t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
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t->rcu_blocked_node = rnp;
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/*
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* If this CPU has already checked in, then this task
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* will hold up the next grace period rather than the
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* current grace period. Queue the task accordingly.
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* If the task is queued for the current grace period
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* (i.e., this CPU has not yet passed through a quiescent
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* state for the current grace period), then as long
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* as that task remains queued, the current grace period
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* cannot end.
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*
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* But first, note that the current CPU must still be
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* on line!
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*/
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WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
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WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
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phase = (rnp->gpnum + !(rnp->qsmask & rdp->grpmask)) & 0x1;
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list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
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spin_unlock_irqrestore(&rnp->lock, flags);
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}
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/*
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* Either we were not in an RCU read-side critical section to
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* begin with, or we have now recorded that critical section
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* globally. Either way, we can now note a quiescent state
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* for this CPU. Again, if we were in an RCU read-side critical
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* section, and if that critical section was blocking the current
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* grace period, then the fact that the task has been enqueued
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* means that we continue to block the current grace period.
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*/
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rcu_preempt_qs(cpu);
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local_irq_save(flags);
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
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local_irq_restore(flags);
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}
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/*
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* Tree-preemptable RCU implementation for rcu_read_lock().
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* Just increment ->rcu_read_lock_nesting, shared state will be updated
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* if we block.
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*/
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void __rcu_read_lock(void)
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{
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ACCESS_ONCE(current->rcu_read_lock_nesting)++;
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barrier(); /* needed if we ever invoke rcu_read_lock in rcutree.c */
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}
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EXPORT_SYMBOL_GPL(__rcu_read_lock);
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/*
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* Check for preempted RCU readers blocking the current grace period
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* for the specified rcu_node structure. If the caller needs a reliable
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* answer, it must hold the rcu_node's ->lock.
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*/
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static int rcu_preempted_readers(struct rcu_node *rnp)
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{
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return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
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}
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static void rcu_read_unlock_special(struct task_struct *t)
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{
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int empty;
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unsigned long flags;
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unsigned long mask;
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struct rcu_node *rnp;
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int special;
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/* NMI handlers cannot block and cannot safely manipulate state. */
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if (in_nmi())
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return;
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local_irq_save(flags);
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/*
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* If RCU core is waiting for this CPU to exit critical section,
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* let it know that we have done so.
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*/
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special = t->rcu_read_unlock_special;
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if (special & RCU_READ_UNLOCK_NEED_QS) {
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
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rcu_preempt_qs(smp_processor_id());
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}
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/* Hardware IRQ handlers cannot block. */
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if (in_irq()) {
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local_irq_restore(flags);
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return;
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}
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/* Clean up if blocked during RCU read-side critical section. */
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if (special & RCU_READ_UNLOCK_BLOCKED) {
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
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/*
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* Remove this task from the list it blocked on. The
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* task can migrate while we acquire the lock, but at
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* most one time. So at most two passes through loop.
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*/
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for (;;) {
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rnp = t->rcu_blocked_node;
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spin_lock(&rnp->lock); /* irqs already disabled. */
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if (rnp == t->rcu_blocked_node)
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break;
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spin_unlock(&rnp->lock); /* irqs remain disabled. */
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}
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empty = !rcu_preempted_readers(rnp);
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list_del_init(&t->rcu_node_entry);
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t->rcu_blocked_node = NULL;
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/*
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* If this was the last task on the current list, and if
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* we aren't waiting on any CPUs, report the quiescent state.
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* Note that both cpu_quiet_msk_finish() and cpu_quiet_msk()
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* drop rnp->lock and restore irq.
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*/
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if (!empty && rnp->qsmask == 0 &&
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!rcu_preempted_readers(rnp)) {
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struct rcu_node *rnp_p;
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if (rnp->parent == NULL) {
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/* Only one rcu_node in the tree. */
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cpu_quiet_msk_finish(&rcu_preempt_state, flags);
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return;
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}
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/* Report up the rest of the hierarchy. */
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mask = rnp->grpmask;
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spin_unlock_irqrestore(&rnp->lock, flags);
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rnp_p = rnp->parent;
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spin_lock_irqsave(&rnp_p->lock, flags);
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WARN_ON_ONCE(rnp->qsmask);
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cpu_quiet_msk(mask, &rcu_preempt_state, rnp_p, flags);
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return;
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}
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spin_unlock(&rnp->lock);
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}
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local_irq_restore(flags);
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}
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/*
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* Tree-preemptable RCU implementation for rcu_read_unlock().
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* Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
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* rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
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* invoke rcu_read_unlock_special() to clean up after a context switch
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* in an RCU read-side critical section and other special cases.
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*/
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void __rcu_read_unlock(void)
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{
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struct task_struct *t = current;
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barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */
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if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 &&
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unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
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rcu_read_unlock_special(t);
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}
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EXPORT_SYMBOL_GPL(__rcu_read_unlock);
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#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
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/*
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* Scan the current list of tasks blocked within RCU read-side critical
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* sections, printing out the tid of each.
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*/
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static void rcu_print_task_stall(struct rcu_node *rnp)
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{
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unsigned long flags;
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struct list_head *lp;
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int phase;
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struct task_struct *t;
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if (rcu_preempted_readers(rnp)) {
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spin_lock_irqsave(&rnp->lock, flags);
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phase = rnp->gpnum & 0x1;
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lp = &rnp->blocked_tasks[phase];
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list_for_each_entry(t, lp, rcu_node_entry)
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printk(" P%d", t->pid);
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spin_unlock_irqrestore(&rnp->lock, flags);
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}
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}
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#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
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/*
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* Check that the list of blocked tasks for the newly completed grace
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* period is in fact empty. It is a serious bug to complete a grace
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* period that still has RCU readers blocked! This function must be
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* invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
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* must be held by the caller.
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*/
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static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
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{
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WARN_ON_ONCE(rcu_preempted_readers(rnp));
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WARN_ON_ONCE(rnp->qsmask);
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}
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#ifdef CONFIG_HOTPLUG_CPU
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/*
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* Handle tasklist migration for case in which all CPUs covered by the
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* specified rcu_node have gone offline. Move them up to the root
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* rcu_node. The reason for not just moving them to the immediate
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* parent is to remove the need for rcu_read_unlock_special() to
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* make more than two attempts to acquire the target rcu_node's lock.
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*
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* Returns 1 if there was previously a task blocking the current grace
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* period on the specified rcu_node structure.
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*
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* The caller must hold rnp->lock with irqs disabled.
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*/
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static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
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struct rcu_node *rnp,
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struct rcu_data *rdp)
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{
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int i;
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struct list_head *lp;
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struct list_head *lp_root;
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int retval = rcu_preempted_readers(rnp);
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struct rcu_node *rnp_root = rcu_get_root(rsp);
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struct task_struct *tp;
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if (rnp == rnp_root) {
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WARN_ONCE(1, "Last CPU thought to be offlined?");
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return 0; /* Shouldn't happen: at least one CPU online. */
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}
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WARN_ON_ONCE(rnp != rdp->mynode &&
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(!list_empty(&rnp->blocked_tasks[0]) ||
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!list_empty(&rnp->blocked_tasks[1])));
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/*
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* Move tasks up to root rcu_node. Rely on the fact that the
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* root rcu_node can be at most one ahead of the rest of the
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* rcu_nodes in terms of gp_num value. This fact allows us to
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* move the blocked_tasks[] array directly, element by element.
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*/
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for (i = 0; i < 2; i++) {
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lp = &rnp->blocked_tasks[i];
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lp_root = &rnp_root->blocked_tasks[i];
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while (!list_empty(lp)) {
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tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
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spin_lock(&rnp_root->lock); /* irqs already disabled */
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list_del(&tp->rcu_node_entry);
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tp->rcu_blocked_node = rnp_root;
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list_add(&tp->rcu_node_entry, lp_root);
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spin_unlock(&rnp_root->lock); /* irqs remain disabled */
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}
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}
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return retval;
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}
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/*
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* Do CPU-offline processing for preemptable RCU.
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*/
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static void rcu_preempt_offline_cpu(int cpu)
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{
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__rcu_offline_cpu(cpu, &rcu_preempt_state);
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}
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#endif /* #ifdef CONFIG_HOTPLUG_CPU */
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/*
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* Check for a quiescent state from the current CPU. When a task blocks,
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* the task is recorded in the corresponding CPU's rcu_node structure,
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* which is checked elsewhere.
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*
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* Caller must disable hard irqs.
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*/
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static void rcu_preempt_check_callbacks(int cpu)
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{
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struct task_struct *t = current;
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if (t->rcu_read_lock_nesting == 0) {
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
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rcu_preempt_qs(cpu);
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return;
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}
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if (per_cpu(rcu_preempt_data, cpu).qs_pending)
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t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
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}
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/*
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* Process callbacks for preemptable RCU.
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*/
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static void rcu_preempt_process_callbacks(void)
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{
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__rcu_process_callbacks(&rcu_preempt_state,
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&__get_cpu_var(rcu_preempt_data));
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}
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/*
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* Queue a preemptable-RCU callback for invocation after a grace period.
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*/
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void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
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{
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__call_rcu(head, func, &rcu_preempt_state);
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}
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EXPORT_SYMBOL_GPL(call_rcu);
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/*
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* Wait for an rcu-preempt grace period. We are supposed to expedite the
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* grace period, but this is the crude slow compatability hack, so just
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* invoke synchronize_rcu().
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*/
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void synchronize_rcu_expedited(void)
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{
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synchronize_rcu();
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}
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EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
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/*
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* Check to see if there is any immediate preemptable-RCU-related work
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* to be done.
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*/
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static int rcu_preempt_pending(int cpu)
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{
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return __rcu_pending(&rcu_preempt_state,
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&per_cpu(rcu_preempt_data, cpu));
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}
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/*
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* Does preemptable RCU need the CPU to stay out of dynticks mode?
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*/
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static int rcu_preempt_needs_cpu(int cpu)
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{
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return !!per_cpu(rcu_preempt_data, cpu).nxtlist;
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}
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/**
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* rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
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*/
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void rcu_barrier(void)
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{
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_rcu_barrier(&rcu_preempt_state, call_rcu);
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}
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EXPORT_SYMBOL_GPL(rcu_barrier);
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/*
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* Initialize preemptable RCU's per-CPU data.
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*/
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static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
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{
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rcu_init_percpu_data(cpu, &rcu_preempt_state, 1);
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}
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/*
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* Move preemptable RCU's callbacks to ->orphan_cbs_list.
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*/
|
|
static void rcu_preempt_send_cbs_to_orphanage(void)
|
|
{
|
|
rcu_send_cbs_to_orphanage(&rcu_preempt_state);
|
|
}
|
|
|
|
/*
|
|
* Initialize preemptable RCU's state structures.
|
|
*/
|
|
static void __init __rcu_init_preempt(void)
|
|
{
|
|
RCU_INIT_FLAVOR(&rcu_preempt_state, rcu_preempt_data);
|
|
}
|
|
|
|
/*
|
|
* Check for a task exiting while in a preemptable-RCU read-side
|
|
* critical section, clean up if so. No need to issue warnings,
|
|
* as debug_check_no_locks_held() already does this if lockdep
|
|
* is enabled.
|
|
*/
|
|
void exit_rcu(void)
|
|
{
|
|
struct task_struct *t = current;
|
|
|
|
if (t->rcu_read_lock_nesting == 0)
|
|
return;
|
|
t->rcu_read_lock_nesting = 1;
|
|
rcu_read_unlock();
|
|
}
|
|
|
|
#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
|
|
|
|
/*
|
|
* Tell them what RCU they are running.
|
|
*/
|
|
static inline void rcu_bootup_announce(void)
|
|
{
|
|
printk(KERN_INFO "Hierarchical RCU implementation.\n");
|
|
}
|
|
|
|
/*
|
|
* Return the number of RCU batches processed thus far for debug & stats.
|
|
*/
|
|
long rcu_batches_completed(void)
|
|
{
|
|
return rcu_batches_completed_sched();
|
|
}
|
|
EXPORT_SYMBOL_GPL(rcu_batches_completed);
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, we never have to check for
|
|
* CPUs being in quiescent states.
|
|
*/
|
|
static void rcu_preempt_note_context_switch(int cpu)
|
|
{
|
|
}
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, there are never any preempted
|
|
* RCU readers.
|
|
*/
|
|
static int rcu_preempted_readers(struct rcu_node *rnp)
|
|
{
|
|
return 0;
|
|
}
|
|
|
|
#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, we never have to check for
|
|
* tasks blocked within RCU read-side critical sections.
|
|
*/
|
|
static void rcu_print_task_stall(struct rcu_node *rnp)
|
|
{
|
|
}
|
|
|
|
#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
|
|
|
|
/*
|
|
* Because there is no preemptable RCU, there can be no readers blocked,
|
|
* so there is no need to check for blocked tasks. So check only for
|
|
* bogus qsmask values.
|
|
*/
|
|
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
|
|
{
|
|
WARN_ON_ONCE(rnp->qsmask);
|
|
}
|
|
|
|
#ifdef CONFIG_HOTPLUG_CPU
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it never needs to migrate
|
|
* tasks that were blocked within RCU read-side critical sections, and
|
|
* such non-existent tasks cannot possibly have been blocking the current
|
|
* grace period.
|
|
*/
|
|
static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
|
|
struct rcu_node *rnp,
|
|
struct rcu_data *rdp)
|
|
{
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it never needs CPU-offline
|
|
* processing.
|
|
*/
|
|
static void rcu_preempt_offline_cpu(int cpu)
|
|
{
|
|
}
|
|
|
|
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it never has any callbacks
|
|
* to check.
|
|
*/
|
|
static void rcu_preempt_check_callbacks(int cpu)
|
|
{
|
|
}
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it never has any callbacks
|
|
* to process.
|
|
*/
|
|
static void rcu_preempt_process_callbacks(void)
|
|
{
|
|
}
|
|
|
|
/*
|
|
* In classic RCU, call_rcu() is just call_rcu_sched().
|
|
*/
|
|
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
|
|
{
|
|
call_rcu_sched(head, func);
|
|
}
|
|
EXPORT_SYMBOL_GPL(call_rcu);
|
|
|
|
/*
|
|
* Wait for an rcu-preempt grace period, but make it happen quickly.
|
|
* But because preemptable RCU does not exist, map to rcu-sched.
|
|
*/
|
|
void synchronize_rcu_expedited(void)
|
|
{
|
|
synchronize_sched_expedited();
|
|
}
|
|
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it never has any work to do.
|
|
*/
|
|
static int rcu_preempt_pending(int cpu)
|
|
{
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it never needs any CPU.
|
|
*/
|
|
static int rcu_preempt_needs_cpu(int cpu)
|
|
{
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, rcu_barrier() is just
|
|
* another name for rcu_barrier_sched().
|
|
*/
|
|
void rcu_barrier(void)
|
|
{
|
|
rcu_barrier_sched();
|
|
}
|
|
EXPORT_SYMBOL_GPL(rcu_barrier);
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, there is no per-CPU
|
|
* data to initialize.
|
|
*/
|
|
static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
|
|
{
|
|
}
|
|
|
|
/*
|
|
* Because there is no preemptable RCU, there are no callbacks to move.
|
|
*/
|
|
static void rcu_preempt_send_cbs_to_orphanage(void)
|
|
{
|
|
}
|
|
|
|
/*
|
|
* Because preemptable RCU does not exist, it need not be initialized.
|
|
*/
|
|
static void __init __rcu_init_preempt(void)
|
|
{
|
|
}
|
|
|
|
#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
|