Move c_src dirs back to toplevel

We just wanted c sourcefiles out of the top level, not c source
directories.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 37 insertions, 0 deletions

diff --git a/linux/preempt.c b/linux/preempt.c
new file mode 100644
index 00000000..72eceed3
--- /dev/null
+++ b/linux/preempt.c
@@ -0,0 +1,37 @@
+#include <pthread.h>
+
+#include "linux/preempt.h"
+
+/*
+ * In userspace, pthreads are preemptible and can migrate CPUs at any time.
+ *
+ * In the kernel, preempt_disable() logic essentially guarantees that a marked
+ * critical section owns its CPU for the relevant block. This is necessary for
+ * various code paths, critically including the percpu system as it allows for
+ * non-atomic reads and writes to CPU-local data structures.
+ *
+ * The high performance userspace equivalent would be to use thread local
+ * storage to replace percpu data, but that would be complicated. It should be
+ * correct to instead guarantee mutual exclusion for the critical sections.
+ */
+
+static pthread_mutex_t preempt_lock;
+
+__attribute__((constructor))
+static void preempt_init(void) {
+	pthread_mutexattr_t attr;
+	pthread_mutexattr_init(&attr);
+	pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+	pthread_mutex_init(&preempt_lock, &attr);
+	pthread_mutexattr_destroy(&attr);
+}
+
+void preempt_disable(void)
+{
+	pthread_mutex_lock(&preempt_lock);
+}
+
+void preempt_enable(void)
+{
+	pthread_mutex_unlock(&preempt_lock);
+}