We had a bug where we forgot to zero the .tbss section (the zero part of

the TLS), which led to preempt_counter not being intialized to zero and
therefore preemption not being enabled for new threads.

Such non-preemptable threads can monopolize their CPU and cause other
threads with the misfortune of being assigned to this CPU to never run.

This patch adds a simple test reproducing this bug. Because we don't have
an implementation of __tls_get_addr(), we can't "extern" a TLS symbol
(here sched::preempt_counter) from the test, and so I had to create a
new sched::get_preempt_counter() function which I can "extern" from the
test.

bootfs.manifest

@@ -69,6 +69,7 @@
 /tests/tst-bsd-netdriver.so: ./tests/tst-bsd-netdriver.so
 /tests/tst-virtionet.so: ./tests/tst-virtionet.so
 /tests/tst-fpu.so: ./tests/tst-fpu.so
+/tests/tst-preempt.so: ./tests/tst-preempt.so
 /tests/tst-tracepoint.so: ./tests/tst-tracepoint.so
 /tests/tst-leak.so: ./tests/tst-leak.so
 /tests/tst-mmap.so: ./tests/tst-mmap.so

build.mak

@@ -87,6 +87,7 @@ tests += tests/tst-vblk.so tests/tst-fat.so tests/tst-romfs.so tests/bench/bench
 tests += tests/tst-bsd-evh.so tests/tst-bsd-callout.so tests/tst-bsd-netisr.so \
          tests/tst-bsd-netdriver.so tests/tst-virtionet.so
 tests += tests/tst-fpu.so
+tests += tests/tst-preempt.so
 tests += tests/tst-tracepoint.so
 tests += tests/tst-hub.so
 tests += tests/tst-leak.so tests/tst-mmap.so

core/sched.cc

@@ -424,6 +424,11 @@ void preempt_enable()
     // FIXME: may need to schedule() here if a high prio thread is waiting
 }
 
+unsigned int get_preempt_counter()
+{
+    return preempt_counter;
+}
+
 void preempt()
 {
     if (!preempt_counter) {

tests/tst-preempt.cc

+#include "sched.hh"
+#include "debug.hh"
+
+namespace sched {
+    // For this to work, we need to implement __tls_get_addr()
+    //extern unsigned int __thread preempt_counter;
+    // It's much easier, to expose the TLS variable via a function:
+    extern unsigned int get_preempt_counter();
+}
+
+int main(int argc, char **argv)
+{
+    debug("Running preemption tests\n");
+
+    // Test 1: check that new threads start preemptable, i.e., with
+    // preempt_counter==0. We had a bug where we didn't zero the .tbss
+    // section, leading to non-zero preempt_counter initialization.
+    assert(sched::get_preempt_counter() == 0);
+
+    auto t1 = new sched::thread([] {
+            assert(sched::get_preempt_counter() == 0);
+    });
+    t1->start();
+    delete t1;
+
+    debug("Preemption tests succeeded\n");
+
+}