Re: Real-Time Preemption and RCU

From: Ingo Molnar
Date: Tue Mar 22 2005 - 08:12:29 EST



* Esben Nielsen <simlo@xxxxxxxxxx> wrote:

> > > +static inline void rcu_read_lock(void)
> > > +{
> > > + preempt_disable();
> > > + __get_cpu_var(rcu_data).active_readers++;
> > > + preempt_enable();
> > > +}
> >
> > this is buggy. Nothing guarantees that we'll do the rcu_read_unlock() on
> > the same CPU, and hence ->active_readers can get out of sync.
> >
>
> Ok, this have to be handled in the mitigration code somehow. I have already
> added an
> current->rcu_read_depth++
> so it ought to be painless. A simple solution would be not to
> mititagrate threads with rcu_read_depth!=0.

could you elaborate?

In any case, see the new PREEMPT_RCU code in the -40-07 patch (and
upwards). I've also attached a separate patch, it should apply cleanly
to 2.6.12-rc1.

Ingo
--- linux/kernel/rcupdate.c.orig
+++ linux/kernel/rcupdate.c
@@ -468,3 +468,36 @@ module_param(maxbatch, int, 0);
EXPORT_SYMBOL(call_rcu);
EXPORT_SYMBOL(call_rcu_bh);
EXPORT_SYMBOL(synchronize_kernel);
+
+#ifdef CONFIG_PREEMPT_RCU
+
+void rcu_read_lock(void)
+{
+ if (current->rcu_read_lock_nesting++ == 0) {
+ current->rcu_data = &get_cpu_var(rcu_data);
+ atomic_inc(&current->rcu_data->active_readers);
+ put_cpu_var(rcu_data);
+ }
+}
+EXPORT_SYMBOL(rcu_read_lock);
+
+void rcu_read_unlock(void)
+{
+ int cpu;
+
+ if (--current->rcu_read_lock_nesting == 0) {
+ atomic_dec(&current->rcu_data->active_readers);
+ /*
+ * Check whether we have reached quiescent state.
+ * Note! This is only for the local CPU, not for
+ * current->rcu_data's CPU [which typically is the
+ * current CPU, but may also be another CPU].
+ */
+ cpu = get_cpu();
+ rcu_qsctr_inc(cpu);
+ put_cpu();
+ }
+}
+EXPORT_SYMBOL(rcu_read_unlock);
+
+#endif
--- linux/include/linux/rcupdate.h.orig
+++ linux/include/linux/rcupdate.h
@@ -99,6 +99,9 @@ struct rcu_data {
struct rcu_head *donelist;
struct rcu_head **donetail;
int cpu;
+#ifdef CONFIG_PREEMPT_RCU
+ atomic_t active_readers;
+#endif
};

DECLARE_PER_CPU(struct rcu_data, rcu_data);
@@ -115,12 +118,18 @@ extern struct rcu_ctrlblk rcu_bh_ctrlblk
static inline void rcu_qsctr_inc(int cpu)
{
struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
- rdp->passed_quiesc = 1;
+#ifdef CONFIG_PREEMPT_RCU
+ if (!atomic_read(&rdp->active_readers))
+#endif
+ rdp->passed_quiesc = 1;
}
static inline void rcu_bh_qsctr_inc(int cpu)
{
struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
- rdp->passed_quiesc = 1;
+#ifdef CONFIG_PREEMPT_RCU
+ if (!atomic_read(&rdp->active_readers))
+#endif
+ rdp->passed_quiesc = 1;
}

static inline int __rcu_pending(struct rcu_ctrlblk *rcp,
@@ -183,14 +192,22 @@ static inline int rcu_pending(int cpu)
*
* It is illegal to block while in an RCU read-side critical section.
*/
-#define rcu_read_lock() preempt_disable()
+#ifdef CONFIG_PREEMPT_RCU
+ extern void rcu_read_lock(void);
+#else
+# define rcu_read_lock preempt_disable()
+#endif

/**
* rcu_read_unlock - marks the end of an RCU read-side critical section.
*
* See rcu_read_lock() for more information.
*/
-#define rcu_read_unlock() preempt_enable()
+#ifdef CONFIG_PREEMPT_RCU
+ extern void rcu_read_unlock(void);
+#else
+# define rcu_read_unlock preempt_enable()
+#endif

/*
* So where is rcu_write_lock()? It does not exist, as there is no
@@ -213,14 +230,16 @@ static inline int rcu_pending(int cpu)
* can use just rcu_read_lock().
*
*/
-#define rcu_read_lock_bh() local_bh_disable()
+//#define rcu_read_lock_bh() local_bh_disable()
+#define rcu_read_lock_bh() { rcu_read_lock(); local_bh_disable(); }

/*
* rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
*
* See rcu_read_lock_bh() for more information.
*/
-#define rcu_read_unlock_bh() local_bh_enable()
+//#define rcu_read_unlock_bh() local_bh_enable()
+#define rcu_read_unlock_bh() { local_bh_enable(); rcu_read_unlock(); }

/**
* rcu_dereference - fetch an RCU-protected pointer in an
--- linux/include/linux/sched.h.orig
+++ linux/include/linux/sched.h
@@ -727,6 +872,10 @@ struct task_struct {
nodemask_t mems_allowed;
int cpuset_mems_generation;
#endif
+#ifdef CONFIG_PREEMPT_RCU
+ int rcu_read_lock_nesting;
+ struct rcu_data *rcu_data;
+#endif
};

static inline pid_t process_group(struct task_struct *tsk)