ring-buffer: Use READ_ONCE() for most tail_page access

As cpu_buffer->tail_page may be modified by interrupts at almost any time,
the flow of logic is very important. Do not let gcc get smart with
re-reading cpu_buffer->tail_page by adding READ_ONCE() around most of its
accesses.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 11 insertions, 7 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c6045a27ba3..ab102e6259bc 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1036,7 +1036,7 @@ static int rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
          * it is, then it is up to us to update the tail
          * pointer.
          */
-        if (tail_page == cpu_buffer->tail_page) {
+        if (tail_page == READ_ONCE(cpu_buffer->tail_page)) {
                 /* Zero the write counter */
                 unsigned long val = old_write & ~RB_WRITE_MASK;
                 unsigned long eval = old_entries & ~RB_WRITE_MASK;
@@ -2036,12 +2036,15 @@ rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
          * the tail page would have moved.
          */
         if (ret == RB_PAGE_NORMAL) {
+                struct buffer_page *buffer_tail_page;
+
+                buffer_tail_page = READ_ONCE(cpu_buffer->tail_page);
                 /*
                  * If the tail had moved passed next, then we need
                  * to reset the pointer.
                  */
-                if (cpu_buffer->tail_page != tail_page &&
-                    cpu_buffer->tail_page != next_page)
+                if (buffer_tail_page != tail_page &&
+                    buffer_tail_page != next_page)
                         rb_head_page_set_normal(cpu_buffer, new_head,
                                                 next_page,
                                                 RB_PAGE_HEAD);
@@ -2362,7 +2365,7 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
         addr = (unsigned long)event;
         addr &= PAGE_MASK;
 
-        bpage = cpu_buffer->tail_page;
+        bpage = READ_ONCE(cpu_buffer->tail_page);
 
         if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
                 unsigned long write_mask =
@@ -2410,7 +2413,7 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
  again:
         max_count = cpu_buffer->nr_pages * 100;
 
-        while (cpu_buffer->commit_page != cpu_buffer->tail_page) {
+        while (cpu_buffer->commit_page != READ_ONCE(cpu_buffer->tail_page)) {
                 if (RB_WARN_ON(cpu_buffer, !(--max_count)))
                         return;
                 if (RB_WARN_ON(cpu_buffer,
@@ -2443,7 +2446,7 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
          * and pushed the tail page forward, we will be left with
          * a dangling commit that will never go forward.
          */
-        if (unlikely(cpu_buffer->commit_page != cpu_buffer->tail_page))
+        if (unlikely(cpu_buffer->commit_page != READ_ONCE(cpu_buffer->tail_page)))
                 goto again;
 }
 
@@ -2699,7 +2702,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
         if (unlikely(info->add_timestamp))
                 info->length += RB_LEN_TIME_EXTEND;
 
-        tail_page = info->tail_page = cpu_buffer->tail_page;
+        /* Don't let the compiler play games with cpu_buffer->tail_page */
+        tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
         write = local_add_return(info->length, &tail_page->write);
 
         /* set write to only the index of the write */