1 files changed, 32 insertions, 16 deletions

diff --git a/litmus/sched_pdumb.c b/litmus/sched_pdumb.c
index c4a123475160..af1076dc9be0 100644
--- a/litmus/sched_pdumb.c
+++ b/litmus/sched_pdumb.c
@@ -19,8 +19,11 @@ struct dumb_cpu {
         int                     blocked;
         raw_spinlock_t          lock;
 };
+
 /* Define AND initialize one dumb_cpu per CPU.
  * Use of this macro is good for safety and performance reasons.
+ * Beware: this macro creates global variables. If two plugins pick
+ * the same name here, this will not compile.
  */
 DEFINE_PER_CPU(struct dumb_cpu, pdumb_cpus);
 
@@ -40,7 +43,7 @@ static long pdumb_activate_plugin(void)
                 cpu = &per_cpu(pdumb_cpus, i);
                 cpu->dumb_task = NULL;
                 cpu->scheduled = 0;
-                cpu->blocked = 0;
+                cpu->blocked   = 0;
         }
 
         printk(KERN_ERR "Activated a dumb plugin!\n");
@@ -55,6 +58,7 @@ static long pdumb_activate_plugin(void)
 static long pdumb_admit_task(struct task_struct *t)
 {
         long ret;
+        unsigned long flags;
         int cpu_num = get_partition(t);
 
         /* Per CPU variables have to be accessed wierd.
@@ -62,22 +66,23 @@ static long pdumb_admit_task(struct task_struct *t)
          */
         struct dumb_cpu *cpu = &per_cpu(pdumb_cpus, cpu_num);
 
-        raw_spin_lock(&cpu->lock);
+        /* We are accessing state atomically, we need a lock and to
+         * disable irqs.
+         */
+        raw_spin_lock_irqsave(&cpu->lock, flags);
 
         if (cpu->dumb_task) {
-                printk(KERN_ERR "Already have a dumb task on %d!\n",
-                       cpu_num);
+                /* Reject the task, causing a failure in userspace */
+                printk(KERN_ERR "Already have a dumb task on %d!\n", cpu_num);
                 ret = -EINVAL;
         } else {
-                printk(KERN_ERR "Taking your dumb task on %d!\n",
-                       cpu_num);
-
-                /* Assign our dumb task! */
+                /* Assign our dumb task */
+                printk(KERN_ERR "Taking your dumb task on %d!\n", cpu_num);
                 cpu->dumb_task = t;
                 ret = 0;
         }
 
-        raw_spin_unlock(&cpu->lock);
+        raw_spin_unlock_irqrestore(&cpu->lock, flags);
         return ret;
 }
 
@@ -89,15 +94,16 @@ static void pdumb_task_new(struct task_struct *t, int on_rq, int running)
 {
         /* Needed to disable interrupts */
         unsigned long flags;
-        /* The macro __get_cpu_var() returns the local cpu variable */
         struct dumb_cpu *cpu = &per_cpu(pdumb_cpus, get_partition(t));
 
-        /* We are accessing state atomically, we need a lock and to
-         * disable irqs.
-         */
         raw_spin_lock_irqsave(&cpu->lock, flags);
 
-        /* The task could already be running in Linux.
+        /* We only admit one task per cpu, this better be it */
+        BUG_ON(cpu->dumb_task != t);
+
+        /* The task could already be running in Linux. For example, it
+         * could have been running, then switched into real-time
+         * mode. This is how all real-time tasks begin execution.
          * Lets just say its running here too.
          */
         if (running) {
@@ -120,6 +126,7 @@ static void pdumb_task_exit(struct task_struct *t)
         raw_spin_lock_irqsave(&cpu->lock, flags);
         cpu->dumb_task = NULL;
         cpu->scheduled = 0;
+        cpu->blocked   = 0;
         raw_spin_unlock_irqrestore(&cpu->lock, flags);
 }
 
@@ -131,6 +138,7 @@ static void pdumb_task_exit(struct task_struct *t)
  */
 static struct task_struct* pdumb_schedule(struct task_struct *prev)
 {
+        /* The macro __get_cpu_var() returns the local cpu variable */
         struct dumb_cpu *cpu = &__get_cpu_var(pdumb_cpus);
         struct task_struct *sched;
 
@@ -140,6 +148,7 @@ static struct task_struct* pdumb_schedule(struct task_struct *prev)
          */
         raw_spin_lock(&cpu->lock);
 
+        /* Switch between dumb_task and nothing arbitrarily */
         if (cpu->scheduled || cpu->blocked || !cpu->dumb_task) {
                 cpu->scheduled = 0;
                 sched = NULL;
@@ -150,9 +159,15 @@ static struct task_struct* pdumb_schedule(struct task_struct *prev)
                 TRACE_TASK(cpu->dumb_task, "Scheduled!\n");
         }
 
+        /* This must be done when the cpu state has been atomically
+         * chosen. This allows litmus to manage some race conditions
+         * with tasks blocking / preempting / releasing without any
+         * work on the plugin's part.
+         */
         sched_state_task_picked();
 
         raw_spin_unlock(&cpu->lock);
+
         return sched;
 }
 
@@ -171,7 +186,7 @@ static void pdumb_task_block(struct task_struct *t)
         TRACE_TASK(t, "Blocked!\n");
 
         raw_spin_lock_irqsave(&cpu->lock, flags);
-        cpu->blocked = 1;
+        cpu->blocked   = 1;
         cpu->scheduled = 0;
         raw_spin_unlock_irqrestore(&cpu->lock, flags);
 }
@@ -188,7 +203,7 @@ static void pdumb_task_wake_up(struct task_struct *t)
         TRACE_TASK(t, "Awoken!\n");
 
         raw_spin_lock_irqsave(&cpu->lock, flags);
-        cpu->blocked = 0;
+        cpu->blocked   = 0;
         cpu->scheduled = 0;
         raw_spin_unlock_irqrestore(&cpu->lock, flags);
 }
@@ -212,6 +227,7 @@ static struct sched_plugin pdumb_plugin __cacheline_aligned_in_smp = {
         .task_exit              = pdumb_task_exit,
 };
 
+
 /**
  * Called when the system boots up.
  */