2 files changed, 133 insertions, 21 deletions

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index e1b7b2173885..f0aa486d131c 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -68,9 +68,38 @@ ring_buffer_event_time_delta(struct ring_buffer_event *event)
         return event->time_delta;
 }
 
+/*
+ * ring_buffer_event_discard can discard any event in the ring buffer.
+ *   it is up to the caller to protect against a reader from
+ *   consuming it or a writer from wrapping and replacing it.
+ *
+ * No external protection is needed if this is called before
+ * the event is commited. But in that case it would be better to
+ * use ring_buffer_discard_commit.
+ *
+ * Note, if an event that has not been committed is discarded
+ * with ring_buffer_event_discard, it must still be committed.
+ */
 void ring_buffer_event_discard(struct ring_buffer_event *event);
 
 /*
+ * ring_buffer_discard_commit will remove an event that has not
+ *   ben committed yet. If this is used, then ring_buffer_unlock_commit
+ *   must not be called on the discarded event. This function
+ *   will try to remove the event from the ring buffer completely
+ *   if another event has not been written after it.
+ *
+ * Example use:
+ *
+ *  if (some_condition)
+ *    ring_buffer_discard_commit(buffer, event);
+ *  else
+ *    ring_buffer_unlock_commit(buffer, event);
+ */
+void ring_buffer_discard_commit(struct ring_buffer *buffer,
+                                struct ring_buffer_event *event);
+
+/*
  * size is in bytes for each per CPU buffer.
  */
 struct ring_buffer *
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 74a11808c282..f935bd5ec3e8 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -205,27 +205,6 @@ static void rb_event_set_padding(struct ring_buffer_event *event)
         event->time_delta = 0;
 }
 
-/**
- * ring_buffer_event_discard - discard an event in the ring buffer
- * @buffer: the ring buffer
- * @event: the event to discard
- *
- * Sometimes a event that is in the ring buffer needs to be ignored.
- * This function lets the user discard an event in the ring buffer
- * and then that event will not be read later.
- *
- * Note, it is up to the user to be careful with this, and protect
- * against races. If the user discards an event that has been consumed
- * it is possible that it could corrupt the ring buffer.
- */
-void ring_buffer_event_discard(struct ring_buffer_event *event)
-{
-        event->type = RINGBUF_TYPE_PADDING;
-        /* time delta must be non zero */
-        if (!event->time_delta)
-                event->time_delta = 1;
-}
-
 static unsigned
 rb_event_data_length(struct ring_buffer_event *event)
 {
@@ -1571,6 +1550,110 @@ int ring_buffer_unlock_commit(struct ring_buffer *buffer,
 EXPORT_SYMBOL_GPL(ring_buffer_unlock_commit);
 
 /**
+ * ring_buffer_event_discard - discard any event in the ring buffer
+ * @event: the event to discard
+ *
+ * Sometimes a event that is in the ring buffer needs to be ignored.
+ * This function lets the user discard an event in the ring buffer
+ * and then that event will not be read later.
+ *
+ * Note, it is up to the user to be careful with this, and protect
+ * against races. If the user discards an event that has been consumed
+ * it is possible that it could corrupt the ring buffer.
+ */
+void ring_buffer_event_discard(struct ring_buffer_event *event)
+{
+        event->type = RINGBUF_TYPE_PADDING;
+        /* time delta must be non zero */
+        if (!event->time_delta)
+                event->time_delta = 1;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_event_discard);
+
+/**
+ * ring_buffer_commit_discard - discard an event that has not been committed
+ * @buffer: the ring buffer
+ * @event: non committed event to discard
+ *
+ * This is similar to ring_buffer_event_discard but must only be
+ * performed on an event that has not been committed yet. The difference
+ * is that this will also try to free the event from the ring buffer
+ * if another event has not been added behind it.
+ *
+ * If another event has been added behind it, it will set the event
+ * up as discarded, and perform the commit.
+ *
+ * If this function is called, do not call ring_buffer_unlock_commit on
+ * the event.
+ */
+void ring_buffer_discard_commit(struct ring_buffer *buffer,
+                                struct ring_buffer_event *event)
+{
+        struct ring_buffer_per_cpu *cpu_buffer;
+        unsigned long new_index, old_index;
+        struct buffer_page *bpage;
+        unsigned long index;
+        unsigned long addr;
+        int cpu;
+
+        /* The event is discarded regardless */
+        ring_buffer_event_discard(event);
+
+        /*
+         * This must only be called if the event has not been
+         * committed yet. Thus we can assume that preemption
+         * is still disabled.
+         */
+        RB_WARN_ON(buffer, !preempt_count());
+
+        cpu = smp_processor_id();
+        cpu_buffer = buffer->buffers[cpu];
+
+        new_index = rb_event_index(event);
+        old_index = new_index + rb_event_length(event);
+        addr = (unsigned long)event;
+        addr &= PAGE_MASK;
+
+        bpage = cpu_buffer->tail_page;
+
+        if (bpage == (void *)addr && rb_page_write(bpage) == old_index) {
+                /*
+                 * This is on the tail page. It is possible that
+                 * a write could come in and move the tail page
+                 * and write to the next page. That is fine
+                 * because we just shorten what is on this page.
+                 */
+                index = local_cmpxchg(&bpage->write, old_index, new_index);
+                if (index == old_index)
+                        goto out;
+        }
+
+        /*
+         * The commit is still visible by the reader, so we
+         * must increment entries.
+         */
+        cpu_buffer->entries++;
+ out:
+        /*
+         * If a write came in and pushed the tail page
+         * we still need to update the commit pointer
+         * if we were the commit.
+         */
+        if (rb_is_commit(cpu_buffer, event))
+                rb_set_commit_to_write(cpu_buffer);
+
+        /*
+         * Only the last preempt count needs to restore preemption.
+         */
+        if (preempt_count() == 1)
+                ftrace_preempt_enable(per_cpu(rb_need_resched, cpu));
+        else
+                preempt_enable_no_resched_notrace();
+
+}
+EXPORT_SYMBOL_GPL(ring_buffer_discard_commit);
+
+/**
  * ring_buffer_write - write data to the buffer without reserving
  * @buffer: The ring buffer to write to.
  * @length: The length of the data being written (excluding the event header)