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rcu: Clean up code based on review feedback from Josh Triplett

Browse source 
These issues identified during an old-fashioned face-to-face code
review extended over many hours.

o	Bury various forms of the "rsp->completed == rsp->gpnum"
	comparison into an rcu_gp_in_progress() function, which has
	the beneficial side-effect of forcing consistent use of
	ACCESS_ONCE().

o	Replace hand-coded arithmetic with DIV_ROUND_UP().

o	Bury several "!list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x01])"
	instances into an rcu_preempted_readers() function, as this
	expression indicates that there are no readers blocked
	within RCU read-side critical sections blocking the current
	grace period.  (Though there might well be similar readers
	blocking the next grace period.)

o	Remove a dangling rcu_restart_cpu() declaration that has
	been dangling for almost 20 minor releases of the kernel.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12537246442687-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Paul E. McKenney 2009-09-23 09:50:41 -07:00 committed by Ingo Molnar
parent 0729e19614
commit fc2219d49e
4 changed files with 46 additions and 40 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

1
include/linux/rcutree.h
View file

@ -85,7 +85,6 @@ static inline void synchronize_rcu_bh_expedited(void)
extern void __rcu_init(void);
extern void rcu_check_callbacks(int cpu, int user);
extern void rcu_restart_cpu(int cpu);
extern long rcu_batches_completed(void);
extern long rcu_batches_completed_bh(void);

29
kernel/rcutree.c
View file

@ -100,6 +100,16 @@ static void __cpuinit rcu_init_percpu_data(int cpu, struct rcu_state *rsp,
#include "rcutree_plugin.h"
/*
* Return true if an RCU grace period is in progress. The ACCESS_ONCE()s
* permit this function to be invoked without holding the root rcu_node
* structure's ->lock, but of course results can be subject to change.
*/
static int rcu_gp_in_progress(struct rcu_state *rsp)
{
return ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum);
}
/*
* Note a quiescent state. Because we do not need to know
* how many quiescent states passed, just if there was at least
@ -173,9 +183,7 @@ cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
static int
cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
{
/* ACCESS_ONCE() because we are accessing outside of lock. */
return *rdp->nxttail[RCU_DONE_TAIL] &&
ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum);
return *rdp->nxttail[RCU_DONE_TAIL] && !rcu_gp_in_progress(rsp);
}
/*
@ -482,7 +490,7 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
spin_lock_irqsave(&rnp->lock, flags);
delta = jiffies - rsp->jiffies_stall;
if (delta < RCU_STALL_RAT_DELAY || rsp->gpnum == rsp->completed) {
if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {
spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
@ -537,8 +545,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
/* We haven't checked in, so go dump stack. */
print_cpu_stall(rsp);
} else if (rsp->gpnum != rsp->completed &&
delta >= RCU_STALL_RAT_DELAY) {
} else if (rcu_gp_in_progress(rsp) && delta >= RCU_STALL_RAT_DELAY) {
/* They had two time units to dump stack, so complain. */
print_other_cpu_stall(rsp);
@ -703,9 +710,9 @@ rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
* hold rnp->lock, as required by rcu_start_gp(), which will release it.
*/
static void cpu_quiet_msk_finish(struct rcu_state *rsp, unsigned long flags)
__releases(rnp->lock)
__releases(rcu_get_root(rsp)->lock)
{
WARN_ON_ONCE(rsp->completed == rsp->gpnum);
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
rsp->completed = rsp->gpnum;
rcu_process_gp_end(rsp, rsp->rda[smp_processor_id()]);
rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
@ -1092,7 +1099,7 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
struct rcu_node *rnp = rcu_get_root(rsp);
u8 signaled;
if (ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum))
if (!rcu_gp_in_progress(rsp))
return; /* No grace period in progress, nothing to force. */
if (!spin_trylock_irqsave(&rsp->fqslock, flags)) {
rsp->n_force_qs_lh++; /* Inexact, can lose counts. Tough! */
@ -1251,7 +1258,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
/* Start a new grace period if one not already started. */
if (ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum)) {
if (!rcu_gp_in_progress(rsp)) {
unsigned long nestflag;
struct rcu_node *rnp_root = rcu_get_root(rsp);
@ -1331,7 +1338,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
}
/* Has an RCU GP gone long enough to send resched IPIs &c? */
if (ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum) &&
if (rcu_gp_in_progress(rsp) &&
((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)) {
rdp->n_rp_need_fqs++;
return 1;

6
kernel/rcutree.h
View file

@ -48,14 +48,14 @@
#elif NR_CPUS <= RCU_FANOUT_SQ
# define NUM_RCU_LVLS 2
# define NUM_RCU_LVL_0 1
# define NUM_RCU_LVL_1 (((NR_CPUS) + RCU_FANOUT - 1) / RCU_FANOUT)
# define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT)
# define NUM_RCU_LVL_2 (NR_CPUS)
# define NUM_RCU_LVL_3 0
#elif NR_CPUS <= RCU_FANOUT_CUBE
# define NUM_RCU_LVLS 3
# define NUM_RCU_LVL_0 1
# define NUM_RCU_LVL_1 (((NR_CPUS) + RCU_FANOUT_SQ - 1) / RCU_FANOUT_SQ)
# define NUM_RCU_LVL_2 (((NR_CPUS) + (RCU_FANOUT) - 1) / (RCU_FANOUT))
# define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_SQ)
# define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT)
# define NUM_RCU_LVL_3 NR_CPUS
#else
# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"

50
kernel/rcutree_plugin.h
View file

@ -150,6 +150,16 @@ void __rcu_read_lock(void)
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);
/*
* Check for preempted RCU readers blocking the current grace period
* for the specified rcu_node structure. If the caller needs a reliable
* answer, it must hold the rcu_node's ->lock.
*/
static int rcu_preempted_readers(struct rcu_node *rnp)
{
return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
}
static void rcu_read_unlock_special(struct task_struct *t)
{
int empty;
@ -196,7 +206,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
break;
spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
empty = list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
empty = !rcu_preempted_readers(rnp);
list_del_init(&t->rcu_node_entry);
t->rcu_blocked_node = NULL;
@ -207,7 +217,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
* drop rnp->lock and restore irq.
*/
if (!empty && rnp->qsmask == 0 &&
list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1])) {
!rcu_preempted_readers(rnp)) {
struct rcu_node *rnp_p;
if (rnp->parent == NULL) {
@ -257,12 +267,12 @@ static void rcu_print_task_stall(struct rcu_node *rnp)
{
unsigned long flags;
struct list_head *lp;
int phase = rnp->gpnum & 0x1;
int phase;
struct task_struct *t;
if (!list_empty(&rnp->blocked_tasks[phase])) {
if (rcu_preempted_readers(rnp)) {
spin_lock_irqsave(&rnp->lock, flags);
phase = rnp->gpnum & 0x1; /* re-read under lock. */
phase = rnp->gpnum & 0x1;
lp = &rnp->blocked_tasks[phase];
list_for_each_entry(t, lp, rcu_node_entry)
printk(" P%d", t->pid);
@ -281,20 +291,10 @@ static void rcu_print_task_stall(struct rcu_node *rnp)
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
WARN_ON_ONCE(!list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]));
WARN_ON_ONCE(rcu_preempted_readers(rnp));
WARN_ON_ONCE(rnp->qsmask);
}
/*
* Check for preempted RCU readers for the specified rcu_node structure.
* If the caller needs a reliable answer, it must hold the rcu_node's
* >lock.
*/
static int rcu_preempted_readers(struct rcu_node *rnp)
{
return !list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
}
#ifdef CONFIG_HOTPLUG_CPU
/*
@ -461,6 +461,15 @@ 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
/*
@ -483,15 +492,6 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
WARN_ON_ONCE(rnp->qsmask);
}
/*
* 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_HOTPLUG_CPU
/*