kern/thread: make thread_current migration-safe

This change removes the current thread member from the run queues, and moves
the responsibility of maintaining it to the architecture specific tcb module.
For the x86 architecture, the TCB functions use a per-CPU pointer that can
be read and set in a single instruction, making it interrupt-safe and thus
migration-safe.

1 files changed, 29 insertions, 16 deletions

diff --git a/kern/thread.c b/kern/thread.c
index 4d5d74ae..fa86a2ab 100644
--- a/kern/thread.c
+++ b/kern/thread.c
@@ -21,6 +21,7 @@
 #include <kern/kmem.h>
 #include <kern/list.h>
 #include <kern/macros.h>
+#include <kern/panic.h>
 #include <kern/param.h>
 #include <kern/sprintf.h>
 #include <kern/stddef.h>
@@ -30,7 +31,15 @@
 #include <machine/cpu.h>
 #include <machine/tcb.h>
 
-struct thread_runq thread_runqs[MAX_CPUS];
+/*
+ * Per processor run queue.
+ */
+struct thread_runq {
+    struct thread *idle;
+    struct list threads;
+} __aligned(CPU_L1_SIZE);
+
+static struct thread_runq thread_runqs[MAX_CPUS];
 
 /*
  * Statically allocating the idle thread structures enables their use as
@@ -52,7 +61,6 @@ thread_runq_init(struct thread_runq *runq, struct thread *idle)
     idle->flags = 0;
     idle->pinned = 1;
     idle->preempt = 1;
-    runq->current = idle;
     runq->idle = idle;
     list_init(&runq->threads);
 }
@@ -81,6 +89,12 @@ thread_runq_dequeue(struct thread_runq *runq)
     return thread;
 }
 
+static inline struct thread_runq *
+thread_runq_local(void)
+{
+    return &thread_runqs[cpu_id()];
+}
+
 void __init
 thread_bootstrap(void)
 {
@@ -88,6 +102,14 @@ thread_bootstrap(void)
 
     for (i = 0; i < ARRAY_SIZE(thread_runqs); i++)
         thread_runq_init(&thread_runqs[i], &thread_idles[i]);
+
+    tcb_set_current(&thread_idles[0].tcb);
+}
+
+void __init
+thread_ap_bootstrap(void)
+{
+    tcb_set_current(&thread_idles[cpu_id()].tcb);
 }
 
 void __init
@@ -102,13 +124,11 @@ thread_setup(void)
 static void
 thread_main(void)
 {
-    struct thread_runq *runq;
     struct thread *thread;
 
     assert(!cpu_intr_enabled());
 
-    runq = thread_runq_local();
-    thread = runq->current;
+    thread = thread_current();
     cpu_intr_enable();
 
     thread->fn(thread->arg);
@@ -218,7 +238,6 @@ thread_run(void)
     if (thread == NULL)
         thread = runq->idle;
 
-    runq->current = thread;
     tcb_load(&thread->tcb);
 }
 
@@ -236,7 +255,7 @@ thread_schedule(void)
         flags = cpu_intr_save();
 
         runq = thread_runq_local();
-        prev = runq->current;
+        prev = thread_current();
         assert(prev != NULL);
 
         if (prev != runq->idle)
@@ -259,13 +278,11 @@ thread_schedule(void)
 void
 thread_intr_schedule(void)
 {
-    struct thread_runq *runq;
     struct thread *thread;
 
     assert(!cpu_intr_enabled());
 
-    runq = thread_runq_local();
-    thread = runq->current;
+    thread = thread_current();
     assert(thread != NULL);
 
     if ((thread->preempt == 0) && (thread->flags & THREAD_RESCHEDULE))
@@ -275,11 +292,9 @@ thread_intr_schedule(void)
 void
 thread_preempt_schedule(void)
 {
-    struct thread_runq *runq;
     struct thread *thread;
 
-    runq = thread_runq_local();
-    thread = runq->current;
+    thread = thread_current();
     assert(thread != NULL);
 
     if ((thread->preempt == 0))
@@ -289,13 +304,11 @@ thread_preempt_schedule(void)
 void
 thread_tick(void)
 {
-    struct thread_runq *runq;
     struct thread *thread;
 
     assert(!cpu_intr_enabled());
 
-    runq = thread_runq_local();
-    thread = runq->current;
+    thread = thread_current();
     assert(thread != NULL);
     thread->flags |= THREAD_RESCHEDULE;
 }