[for-next][PATCH 4/7] ring-buffer: Use READ_ONCE() for most tail_page access

Steven Rostedt
2015-11-25T16:11:31Z

[-- Attachment #1: 0004-ring-buffer-Use-READ_ONCE-for-most-tail_page-access.patch --]
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From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

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>
---
kernel/trace/ring_buffer.c | 18 +++++++++++-------
1 file 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 */
--
2.6.1