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>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 9c6045a..ab102e6 100644 (file)
--- 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 */