rcu: Simplify rcu_read_unlock_special() quiescent-state accounting
The earlier approach required two scheduling-clock ticks to note an preemptable-RCU quiescent state in the situation in which the scheduling-clock interrupt is unlucky enough to always interrupt an RCU read-side critical section. With this change, the quiescent state is instead noted by the outermost rcu_read_unlock() immediately following the first scheduling-clock tick, or, alternatively, by the first subsequent context switch. Therefore, this change also speeds up grace periods. Suggested-by: Josh Triplett <josh@joshtriplett.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu LKML-Reference: <12528585111945-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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b0e165c035
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3 changed files with 32 additions and 38 deletions
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@ -1740,7 +1740,6 @@ extern cputime_t task_gtime(struct task_struct *p);
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#define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
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#define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
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#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
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#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
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#define RCU_READ_UNLOCK_GOT_QS (1 << 2) /* CPU has responded to RCU core. */
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static inline void rcu_copy_process(struct task_struct *p)
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static inline void rcu_copy_process(struct task_struct *p)
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{
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{
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@ -107,27 +107,23 @@ static void __cpuinit rcu_init_percpu_data(int cpu, struct rcu_state *rsp,
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*/
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*/
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void rcu_sched_qs(int cpu)
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void rcu_sched_qs(int cpu)
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{
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{
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unsigned long flags;
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struct rcu_data *rdp;
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struct rcu_data *rdp;
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local_irq_save(flags);
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rdp = &per_cpu(rcu_sched_data, cpu);
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rdp = &per_cpu(rcu_sched_data, cpu);
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rdp->passed_quiesc = 1;
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rdp->passed_quiesc_completed = rdp->completed;
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rdp->passed_quiesc_completed = rdp->completed;
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rcu_preempt_qs(cpu);
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barrier();
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local_irq_restore(flags);
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rdp->passed_quiesc = 1;
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rcu_preempt_note_context_switch(cpu);
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}
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}
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void rcu_bh_qs(int cpu)
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void rcu_bh_qs(int cpu)
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{
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{
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unsigned long flags;
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struct rcu_data *rdp;
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struct rcu_data *rdp;
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local_irq_save(flags);
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rdp = &per_cpu(rcu_bh_data, cpu);
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rdp = &per_cpu(rcu_bh_data, cpu);
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rdp->passed_quiesc = 1;
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rdp->passed_quiesc_completed = rdp->completed;
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rdp->passed_quiesc_completed = rdp->completed;
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local_irq_restore(flags);
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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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#ifdef CONFIG_NO_HZ
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#ifdef CONFIG_NO_HZ
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@ -615,6 +611,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
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/* Advance to a new grace period and initialize state. */
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/* Advance to a new grace period and initialize state. */
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rsp->gpnum++;
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rsp->gpnum++;
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WARN_ON_ONCE(rsp->signaled == RCU_GP_INIT);
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rsp->signaled = RCU_GP_INIT; /* Hold off force_quiescent_state. */
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rsp->signaled = RCU_GP_INIT; /* Hold off force_quiescent_state. */
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rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
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rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
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record_gp_stall_check_time(rsp);
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record_gp_stall_check_time(rsp);
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@ -64,34 +64,42 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
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* not in a quiescent state. There might be any number of tasks blocked
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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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* while in an RCU read-side critical section.
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*/
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*/
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static void rcu_preempt_qs_record(int cpu)
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static void rcu_preempt_qs(int cpu)
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{
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{
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struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
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struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
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rdp->passed_quiesc = 1;
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rdp->passed_quiesc_completed = rdp->completed;
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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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/*
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/*
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* We have entered the scheduler or are between softirqs in ksoftirqd.
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* We have entered the scheduler, and the current task might soon be
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* If we are in an RCU read-side critical section, we need to reflect
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* context-switched away from. If this task is in an RCU read-side
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* that in the state of the rcu_node structure corresponding to this CPU.
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* critical section, we will no longer be able to rely on the CPU to
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* Caller must disable hardirqs.
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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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*/
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static void rcu_preempt_qs(int cpu)
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static void rcu_preempt_note_context_switch(int cpu)
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{
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{
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struct task_struct *t = current;
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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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int phase;
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struct rcu_data *rdp;
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struct rcu_data *rdp;
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struct rcu_node *rnp;
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struct rcu_node *rnp;
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if (t->rcu_read_lock_nesting &&
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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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(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
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WARN_ON_ONCE(cpu != smp_processor_id());
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/* Possibly blocking in an RCU read-side critical section. */
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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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rdp = rcu_preempt_state.rda[cpu];
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rnp = rdp->mynode;
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rnp = rdp->mynode;
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spin_lock(&rnp->lock);
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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_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
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t->rcu_blocked_node = rnp;
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t->rcu_blocked_node = rnp;
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@ -112,7 +120,7 @@ static void rcu_preempt_qs(int cpu)
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phase = !(rnp->qsmask & rdp->grpmask) ^ (rnp->gpnum & 0x1);
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phase = !(rnp->qsmask & rdp->grpmask) ^ (rnp->gpnum & 0x1);
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list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
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list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
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smp_mb(); /* Ensure later ctxt swtch seen after above. */
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smp_mb(); /* Ensure later ctxt swtch seen after above. */
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spin_unlock(&rnp->lock);
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spin_unlock_irqrestore(&rnp->lock, flags);
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}
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}
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/*
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/*
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@ -124,9 +132,8 @@ static void rcu_preempt_qs(int cpu)
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* grace period, then the fact that the task has been enqueued
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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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* means that we continue to block the current grace period.
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*/
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*/
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rcu_preempt_qs_record(cpu);
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rcu_preempt_qs(cpu);
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t->rcu_read_unlock_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_READ_UNLOCK_GOT_QS);
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}
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}
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/*
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/*
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@ -162,7 +169,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
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special = t->rcu_read_unlock_special;
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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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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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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
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t->rcu_read_unlock_special |= RCU_READ_UNLOCK_GOT_QS;
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rcu_preempt_qs(smp_processor_id());
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}
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}
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/* Hardware IRQ handlers cannot block. */
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/* Hardware IRQ handlers cannot block. */
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@ -199,9 +206,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
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*/
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*/
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if (!empty && rnp->qsmask == 0 &&
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if (!empty && rnp->qsmask == 0 &&
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list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1])) {
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list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1])) {
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t->rcu_read_unlock_special &=
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
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~(RCU_READ_UNLOCK_NEED_QS |
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RCU_READ_UNLOCK_GOT_QS);
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if (rnp->parent == NULL) {
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if (rnp->parent == NULL) {
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/* Only one rcu_node in the tree. */
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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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cpu_quiet_msk_finish(&rcu_preempt_state, flags);
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@ -352,19 +357,12 @@ static void rcu_preempt_check_callbacks(int cpu)
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struct task_struct *t = current;
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struct task_struct *t = current;
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if (t->rcu_read_lock_nesting == 0) {
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if (t->rcu_read_lock_nesting == 0) {
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t->rcu_read_unlock_special &=
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
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~(RCU_READ_UNLOCK_NEED_QS | RCU_READ_UNLOCK_GOT_QS);
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rcu_preempt_qs(cpu);
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rcu_preempt_qs_record(cpu);
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return;
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return;
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}
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}
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if (per_cpu(rcu_preempt_data, cpu).qs_pending) {
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if (per_cpu(rcu_preempt_data, cpu).qs_pending) {
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if (t->rcu_read_unlock_special & RCU_READ_UNLOCK_GOT_QS) {
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t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
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rcu_preempt_qs_record(cpu);
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t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_GOT_QS;
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} else if (!(t->rcu_read_unlock_special &
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RCU_READ_UNLOCK_NEED_QS)) {
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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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}
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}
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}
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@ -451,7 +449,7 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
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* Because preemptable RCU does not exist, we never have to check for
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* Because preemptable RCU does not exist, we never have to check for
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* CPUs being in quiescent states.
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* CPUs being in quiescent states.
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*/
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*/
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static void rcu_preempt_qs(int cpu)
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static void rcu_preempt_note_context_switch(int cpu)
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{
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{
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}
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}
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