/*
* Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition
* Internal non-public definitions that provide either classic
* or preemptible semantics.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*
* Copyright (c) 2010 Linaro
*
* Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
*/
#ifdef CONFIG_TINY_PREEMPT_RCU
#include <linux/delay.h>
/* Global control variables for preemptible RCU. */
struct rcu_preempt_ctrlblk {
struct rcu_ctrlblk rcb; /* curtail: ->next ptr of last CB for GP. */
struct rcu_head **nexttail;
/* Tasks blocked in a preemptible RCU */
/* read-side critical section while an */
/* preemptible-RCU grace period is in */
/* progress must wait for a later grace */
/* period. This pointer points to the */
/* ->next pointer of the last task that */
/* must wait for a later grace period, or */
/* to &->rcb.rcucblist if there is no */
/* such task. */
struct list_head blkd_tasks;
/* Tasks blocked in RCU read-side critical */
/* section. Tasks are placed at the head */
/* of this list and age towards the tail. */
struct list_head *gp_tasks;
/* Pointer to the first task blocking the */
/* current grace period, or NULL if there */
/* is not such task. */
struct list_head *exp_tasks;
/* Pointer to first task blocking the */
/* current expedited grace period, or NULL */
/* if there is no such task. If there */
/* is no current expedited grace period, */
/* then there cannot be any such task. */
u8 gpnum; /* Current grace period. */
u8 gpcpu; /* Last grace period blocked by the CPU. */
u8 completed; /* Last grace period completed. */
/* If all three are equal, RCU is idle. */
};
static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
.rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
.rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
.nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
.blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),
};
static int rcu_preempted_readers_exp(void);
static void rcu_report_exp_done(void);
/*
* Return true if the CPU has not yet responded to the current grace period.
*/
static int rcu_cpu_blocking_cur_gp(void)
{
return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
}
/*
* Check for a running RCU reader. Because there is only one CPU,
* there can be but one running RCU reader at a time. ;-)
*/
static int rcu_preempt_running_reader(void)
{
return current->rcu_read_lock_nesting;
}
/*
* Check for preempted RCU readers blocking any grace period.
* If the caller needs a reliable answer, it must disable hard irqs.
*/
static int rcu_preempt_blocked_readers_any(void)
{
return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
}
/*
* Check for preempted RCU readers blocking the current grace period.
* If the caller needs a reliable answer, it must disable hard irqs.
*/
static int rcu_preempt_blocked_readers_cgp(void)
{
return rcu_preempt_ctrlblk.gp_tasks != NULL;
}
/*
* Return true if another preemptible-RCU grace period is needed.
*/
static int rcu_preempt_needs_another_gp(void)
{
return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
}
/*
* Return true if a preemptible-RCU grace period is in progress.
* The caller must disable hardirqs.
*/
static int rcu_preempt_gp_in_progress(void)
{
return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
}
/*
* Record a preemptible-RCU quiescent state for the specified CPU. Note
* that this just means that the task currently running on the CPU is
* in a quiescent state. There might be any number of tasks blocked
* while in an RCU read-side critical section.
*
* Unlike the other rcu_*_qs() functions, callers to this function
* must disable irqs in order to protect the assignment to
* ->rcu_read_unlock_special.
*
* Because this is a single-CPU implementation, the only way a grace
* period can end is if the CPU is in a quiescent state. The reason is
* that a blocked preemptible-RCU reader can exit its critical section
* only if the CPU is running it at the time. Therefore, when the
* last task blocking the current grace period exits its RCU read-side
* critical section, neither the CPU nor blocked tasks will be stopping
* the current grace period. (In contrast, SMP implementations
* might have CPUs running in RCU read-side critical sections that
* block later grace periods -- but this is not possible given only
* one CPU.)
*/
static void rcu_preempt_cpu_qs(void)
{
/* Record both CPU and task as having responded to current GP. */
rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
/*
* If there is no GP, or if blocked readers are still blocking GP,
* then there is nothing more to do.
*/
if (!rcu_preempt_gp_in_progress() || rcu_preempt_blocked_readers_cgp())
return;
/* Advance callbacks. */
rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.rcb.curtail;
rcu_preempt_ctrlblk.rcb.curtail = rcu_preempt_ctrlblk.nexttail;
/* If there are no blocked readers, next GP is done instantly. */
if (!rcu_preempt_blocked_readers_any())
rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.nexttail;
/* If there are done callbacks, make RCU_SOFTIRQ process them. */
if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
raise_softirq(RCU_SOFTIRQ);
}
/*
* Start a new RCU grace period if warranted. Hard irqs must be disabled.
*/
static void rcu_preempt_start_gp(void)
{
if (!rcu_preempt_gp_in_progress() && rcu_preempt_needs_another_gp()) {
/* Official start of GP. */
rcu_preempt_ctrlblk.gpnum++;
/* Any blocked RCU readers block new GP. */
if (rcu_preempt_blocked_readers_any())
rcu_preempt_ctrlblk.gp_tasks =
rcu_preempt_ctrlblk.blkd_tasks.next;
/* If there is no running reader, CPU is done with GP. */
if (!rcu_preempt_running_reader())
rcu_preempt_cpu_qs();
}
}
/*
* We have entered the scheduler, and the current task might soon be
* context-switched away from. If this task is in an RCU read-side
* critical section, we will no longer be able to rely on the CPU to
* record that fact, so we enqueue the task on the blkd_tasks list.
* If the task started after the current grace period began, as recorded
* by ->gpcpu, we enqueue at the beginning of the list. Otherwise
* before the element referenced by ->gp_tasks (or at the tail if
* ->gp_tasks is NULL) and point ->gp_tasks at the newly added element.
* The task will dequeue itself when it exits the outermost enclosing
* RCU read-side critical section. Therefore, the current grace period
* cannot be permitted to complete until the ->gp_tasks pointer becomes
* NULL.
*
* Caller must disable preemption.
*/
void rcu_preempt_note_context_switch(void)
{
struct task_struct *t = current;
unsigned long flags;
local_irq_save(flags); /* must exclude scheduler_tick(). */
if (rcu_preempt_running_reader() &&
(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
/* Possibly blocking in an RCU read-side critical section. */
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
/*
* If this CPU has already checked in, then this task
* will hold up the next grace period rather than the
* current grace period. Queue the task accordingly.
* If the task is queued for the current grace period
* (i.e., this CPU has not yet passed through a quiescent
* state for the current grace period), then as long
* as that task remains queued, the current grace period
* cannot end.
*/
list_add(&t->rcu_node_entry, &rcu_preempt_ctrlblk.blkd_tasks);
if (rcu_cpu_blocking_cur_gp())
rcu_preempt_ctrlblk.gp_tasks = &t->rcu_node_entry;
}
/*
* Either we were not in an RCU read-side critical section to
* begin with, or we have now recorded that critical section
* globally. Either way, we can now note a quiescent state
* for this CPU. Again, if we were in an RCU read-side critical
* section, and if that critical section was blocking the current
* grace period, then the fact that the task has been enqueued
* means that current grace period continues to be blocked.
*/
rcu_preempt_cpu_qs();
local_irq_restore(flags);
}
/*
* Tiny-preemptible RCU implementation for rcu_read_lock().
* Just increment ->rcu_read_lock_nesting, shared state will be updated
* if we block.
*/
void __rcu_read_lock(void)
{
current->rcu_read_lock_nesting++;
barrier(); /* needed if we ever invoke rcu_read_lock in rcutiny.c */
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);
/*
* Handle special cases during rcu_read_unlock(), such as needing to
* notify RCU core processing or task having blocked during the RCU
* read-side critical section.
*/
static void rcu_read_unlock_special(struct task_struct *t)
{
int empty;
int empty_exp;
unsigned long flags;
struct list_head *np;
int special;
/*
* NMI handlers cannot block and cannot safely manipulate state.
* They therefore cannot possibly be special, so just leave.
*/
if (in_nmi())
return;
local_irq_save(flags);
/*
* If RCU core is waiting for this CPU to exit critical section,
* let it know that we have done so.
*/
special = t->rcu_read_unlock_special;
if (special & RCU_READ_UNLOCK_NEED_QS)
rcu_preempt_cpu_qs();
/* Hardware IRQ handlers cannot block. */
if (in_irq()) {
local_irq_restore(flags);
return;
}
/* Clean up if blocked during RCU read-side critical section. */
if (special & RCU_READ_UNLOCK_BLOCKED) {
t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
/*
* Remove this task from the ->blkd_tasks list and adjust
* any pointers that might have been referencing it.
*/
empty = !rcu_preempt_blocked_readers_cgp();
empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
np = t->rcu_node_entry.next;
if (np == &rcu_preempt_ctrlblk.blkd_tasks)
np = NULL;
list_del(&t->rcu_node_entry);
if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
rcu_preempt_ctrlblk.gp_tasks = np;
if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
rcu_preempt_ctrlblk.exp_tasks = np;
INIT_LIST_HEAD(&t->rcu_node_entry);
/*
* If this was the last task on the current list, and if
* we aren't waiting on the CPU, report the quiescent state
* and start a new grace period if needed.
*/
if (!empty && !rcu_preempt_blocked_readers_cgp()) {
rcu_preempt_cpu_qs();
rcu_preempt_start_gp();
}
/*
* If this was the last task on the expedited lists,
* then we need wake up the waiting task.
*/
if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
rcu_report_exp_done();
}
local_irq_restore(flags);
}
/*
* Tiny-preemptible RCU implementation for rcu_read_unlock().
* Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
* rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
* invoke rcu_read_unlock_special() to clean up after a context switch
* in an RCU read-side critical section and other special cases.
*/
void __rcu_read_unlock(void)
{
struct task_struct *t = current;
barrier(); /* needed if we ever invoke rcu_read_unlock in rcutiny.c */
--t->rcu_read_lock_nesting;
barrier(); /* decrement before load of ->rcu_read_unlock_special */
if (t->rcu_read_lock_nesting == 0 &&
unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
rcu_read_unlock_special(t);
#ifdef CONFIG_PROVE_LOCKING
WARN_ON_ONCE(t->rcu_read_lock_nesting < 0);
#endif /* #ifdef CONFIG_PROVE_LOCKING */
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);
/*
* Check for a quiescent state from the current CPU. When a task blocks,
* the task is recorded in the rcu_preempt_ctrlblk structure, which is
* checked elsewhere. This is called from the scheduling-clock interrupt.
*
* Caller must disable hard irqs.
*/
static void rcu_preempt_check_callbacks(void)
{
struct task_struct *t = current;
if (rcu_preempt_gp_in_progress() &&
(!rcu_preempt_running_reader() ||
!rcu_cpu_blocking_cur_gp()))
rcu_preempt_cpu_qs();
if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
rcu_preempt_ctrlblk.rcb.donetail)
raise_softirq(RCU_SOFTIRQ);
if (rcu_preempt_gp_in_progress() &&
rcu_cpu_blocking_cur_gp() &&
rcu_preempt_running_reader())
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
}
/*
* TINY_PREEMPT_RCU has an extra callback-list tail pointer to
* update, so this is invoked from __rcu_process_callbacks() to
* handle that case. Of course, it is invoked for all flavors of
* RCU, but RCU callbacks can appear only on one of the lists, and
* neither ->nexttail nor ->donetail can possibly be NULL, so there
* is no need for an explicit check.
*/
static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
{
if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
}
/*
* Process callbacks for preemptible RCU.
*/
static void rcu_preempt_process_callbacks(void)
{
__rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
}
/*
* Queue a preemptible -RCU callback for invocation after a grace period.
*/
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
unsigned long flags;
debug_rcu_head_queue(head);
head->func = func;
head->next = NULL;
local_irq_save(flags);
*rcu_preempt_ctrlblk.nexttail = head;
rcu_preempt_ctrlblk.nexttail = &head->next;
rcu_preempt_start_gp(); /* checks to see if GP needed. */
local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu);
void rcu_barrier(void)
{
struct rcu_synchronize rcu;
init_rcu_head_on_stack(&rcu.head);
init_completion(&rcu.completion);
/* Will wake me after RCU finished. */
call_rcu(&rcu.head, wakeme_after_rcu);
/* Wait for it. */
wait_for_completion(&rcu.completion);
destroy_rcu_head_on_stack(&rcu.head);
}
EXPORT_SYMBOL_GPL(rcu_barrier);
/*
* synchronize_rcu - wait until a grace period has elapsed.
*
* Control will return to the caller some time after a full grace
* period has elapsed, in other words after all currently executing RCU
* read-side critical sections have completed. RCU read-side critical
* sections are delimited by rcu_read_lock() and rcu_read_unlock(),
* and may be nested.
*/
void synchronize_rcu(void)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
if (!rcu_scheduler_active)
return;
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
WARN_ON_ONCE(rcu_preempt_running_reader());
if (!rcu_preempt_blocked_readers_any())
return;
/* Once we get past the fastpath checks, same code as rcu_barrier(). */
rcu_barrier();
}
EXPORT_SYMBOL_GPL(synchronize_rcu);
static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
static unsigned long sync_rcu_preempt_exp_count;
static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
/*
* Return non-zero if there are any tasks in RCU read-side critical
* sections blocking the current preemptible-RCU expedited grace period.
* If there is no preemptible-RCU expedited grace period currently in
* progress, returns zero unconditionally.
*/
static int rcu_preempted_readers_exp(void)
{
return rcu_preempt_ctrlblk.exp_tasks != NULL;
}
/*
* Report the exit from RCU read-side critical section for the last task
* that queued itself during or before the current expedited preemptible-RCU
* grace period.
*/
static void rcu_report_exp_done(void)
{
wake_up(&sync_rcu_preempt_exp_wq);
}
/*
* Wait for an rcu-preempt grace period, but expedite it. The basic idea
* is to rely in the fact that there is but one CPU, and that it is
* illegal for a task to invoke synchronize_rcu_expedited() while in a
* preemptible-RCU read-side critical section. Therefore, any such
* critical sections must correspond to blocked tasks, which must therefore
* be on the ->blkd_tasks list. So just record the current head of the
* list in the ->exp_tasks pointer, and wait for all tasks including and
* after the task pointed to by ->exp_tasks to drain.
*/
void synchronize_rcu_expedited(void)
{
unsigned long flags;
struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
unsigned long snap;
barrier(); /* ensure prior action seen before grace period. */
WARN_ON_ONCE(rcu_preempt_running_reader());
/*
* Acquire lock so that there is only one preemptible RCU grace
* period in flight. Of course, if someone does the expedited
* grace period for us while we are acquiring the lock, just leave.
*/
snap = sync_rcu_preempt_exp_count + 1;
mutex_lock(&sync_rcu_preempt_exp_mutex);
if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
goto unlock_mb_ret; /* Others did our work for us. */
local_irq_save(flags);
/*
* All RCU readers have to already be on blkd_tasks because
* we cannot legally be executing in an RCU read-side critical
* section.
*/
/* Snapshot current head of ->blkd_tasks list. */
rpcp->exp_tasks = rpcp->blkd_tasks.next;
if (rpcp->exp_tasks == &rpcp->blkd_tasks)
rpcp->exp_tasks = NULL;
local_irq_restore(flags);
/* Wait for tail of ->blkd_tasks list to drain. */
if (rcu_preempted_readers_exp())
wait_event(sync_rcu_preempt_exp_wq,
!rcu_preempted_readers_exp());
/* Clean up and exit. */
barrier(); /* ensure expedited GP seen before counter increment. */
sync_rcu_preempt_exp_count++;
unlock_mb_ret:
mutex_unlock(&sync_rcu_preempt_exp_mutex);
barrier(); /* ensure subsequent action seen after grace period. */
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
/*
* Does preemptible RCU need the CPU to stay out of dynticks mode?
*/
int rcu_preempt_needs_cpu(void)
{
if (!rcu_preempt_running_reader())
rcu_preempt_cpu_qs();
return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
}
/*
* Check for a task exiting while in a preemptible -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_TINY_PREEMPT_RCU */
/*
* Because preemptible RCU does not exist, it never has any callbacks
* to check.
*/
static void rcu_preempt_check_callbacks(void)
{
}
/*
* Because preemptible RCU does not exist, it never has any callbacks
* to remove.
*/
static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
{
}
/*
* Because preemptible RCU does not exist, it never has any callbacks
* to process.
*/
static void rcu_preempt_process_callbacks(void)
{
}
#endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */
#ifdef CONFIG_DEBUG_LOCK_ALLOC
#include <linux/kernel_stat.h>
/*
* During boot, we forgive RCU lockdep issues. After this function is
* invoked, we start taking RCU lockdep issues seriously.
*/
void rcu_scheduler_starting(void)
{
WARN_ON(nr_context_switches() > 0);
rcu_scheduler_active = 1;
}
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
