1 files changed, 118 insertions, 0 deletions

diff --git a/include/litmus/feather_buffer.h b/include/litmus/feather_buffer.h
new file mode 100644
index 000000000000..38de95b73553
--- /dev/null
+++ b/include/litmus/feather_buffer.h
@@ -0,0 +1,118 @@
+#ifndef _FEATHER_BUFFER_H_
+#define _FEATHER_BUFFER_H_
+
+/* requires UINT_MAX and memcpy */
+
+#define SLOT_FREE       0
+#define SLOT_BUSY       1
+#define SLOT_READY      2
+
+struct ft_buffer {
+        unsigned int    slot_count;
+        unsigned int    slot_size;
+
+        int             free_count;
+        unsigned int    write_idx;
+        unsigned int    read_idx;
+
+        char*           slots;
+        void*           buffer_mem;
+        unsigned int    failed_writes;
+};
+
+static inline int init_ft_buffer(struct ft_buffer*      buf,
+                                 unsigned int           slot_count,
+                                 unsigned int           slot_size,
+                                 char*                  slots,
+                                 void*                  buffer_mem)
+{
+        int i = 0;
+        if (!slot_count || UINT_MAX % slot_count != slot_count - 1) {
+                /* The slot count must divide UNIT_MAX + 1 so that when it
+                 * wraps around the index correctly points to 0.
+                 */
+                return 0;
+        } else {
+                buf->slot_count    = slot_count;
+                buf->slot_size     = slot_size;
+                buf->slots         = slots;
+                buf->buffer_mem    = buffer_mem;
+                buf->free_count    = slot_count;
+                buf->write_idx     = 0;
+                buf->read_idx      = 0;
+                buf->failed_writes = 0;
+                for (i = 0; i < slot_count; i++)
+                        buf->slots[i] = SLOT_FREE;
+                return 1;
+        }
+}
+
+static inline int ft_buffer_start_write(struct ft_buffer* buf, void **ptr)
+{
+        int free = fetch_and_dec(&buf->free_count);
+        unsigned int idx;
+        if (free <= 0) {
+                fetch_and_inc(&buf->free_count);
+                *ptr = 0;
+                fetch_and_inc(&buf->failed_writes);
+                return 0;
+        } else {
+                idx  = fetch_and_inc((int*) &buf->write_idx) % buf->slot_count;
+                buf->slots[idx] = SLOT_BUSY;
+                *ptr = ((char*) buf->buffer_mem) + idx * buf->slot_size;
+                return 1;
+        }
+}
+
+/* For single writer scenarios, with fewer atomic ops. */
+static inline int ft_buffer_start_single_write(struct ft_buffer* buf, void **ptr)
+{
+        unsigned int idx;
+
+        if (buf->free_count <= 0) {
+                *ptr = 0;
+                /* single writer: no atomicity needed */
+                buf->failed_writes++;
+                return 0;
+        } else {
+                /* free_count is positive, and can only increase since we are
+                 * (by assumption) the only writer accessing the buffer.
+                 */
+
+                idx  = buf->write_idx++ % buf->slot_count;
+                buf->slots[idx] = SLOT_BUSY;
+                *ptr = ((char*) buf->buffer_mem) + idx * buf->slot_size;
+
+                ft_atomic_dec(&buf->free_count);
+                return 1;
+        }
+}
+
+static inline void ft_buffer_finish_write(struct ft_buffer* buf, void *ptr)
+{
+        unsigned int idx = ((char*) ptr - (char*) buf->buffer_mem) / buf->slot_size;
+        buf->slots[idx]  = SLOT_READY;
+}
+
+
+/* exclusive reader access is assumed */
+static inline int ft_buffer_read(struct ft_buffer* buf, void* dest)
+{
+        unsigned int idx;
+        if (buf->free_count == buf->slot_count)
+                /* nothing available */
+                return 0;
+        idx = buf->read_idx % buf->slot_count;
+        if (buf->slots[idx] == SLOT_READY) {
+                memcpy(dest, ((char*) buf->buffer_mem) + idx * buf->slot_size,
+                       buf->slot_size);
+                buf->slots[idx] = SLOT_FREE;
+                buf->read_idx++;
+                fetch_and_inc(&buf->free_count);
+                return 1;
+        } else
+                return 0;
+}
+
+
+#endif