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Subject[RFC][PATCH 05/11] ring-buffer: Have rb_iter_head_event() handle concurrent writer
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

Have the ring_buffer_iter structure have a place to store an event, such
that it can not be overwritten by a writer, and load it in such a way via
rb_iter_head_event() that it will return NULL and reset the iter to the
start of the current page if a writer updated the page.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 106 ++++++++++++++++++++++++++-----------
1 file changed, 75 insertions(+), 31 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index e689bdcb53e8..3d718add73c1 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -503,11 +503,13 @@ struct trace_buffer {
struct ring_buffer_iter {
struct ring_buffer_per_cpu *cpu_buffer;
unsigned long head;
+ unsigned long next_event;
struct buffer_page *head_page;
struct buffer_page *cache_reader_page;
unsigned long cache_read;
u64 read_stamp;
u64 page_stamp;
+ struct ring_buffer_event *event;
};

/**
@@ -1914,15 +1916,59 @@ rb_reader_event(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->reader_page->read);
}

-static __always_inline struct ring_buffer_event *
-rb_iter_head_event(struct ring_buffer_iter *iter)
+static __always_inline unsigned rb_page_commit(struct buffer_page *bpage)
{
- return __rb_page_index(iter->head_page, iter->head);
+ return local_read(&bpage->page->commit);
}

-static __always_inline unsigned rb_page_commit(struct buffer_page *bpage)
+static struct ring_buffer_event *
+rb_iter_head_event(struct ring_buffer_iter *iter)
{
- return local_read(&bpage->page->commit);
+ struct ring_buffer_event *event;
+ struct buffer_page *iter_head_page = iter->head_page;
+ unsigned long commit;
+ unsigned length;
+
+ /*
+ * When the writer goes across pages, it issues a cmpxchg which
+ * is a mb(), which will synchronize with the rmb here.
+ * (see rb_tail_page_update() and __rb_reserve_next())
+ */
+ commit = rb_page_commit(iter_head_page);
+ smp_rmb();
+ event = __rb_page_index(iter_head_page, iter->head);
+ length = rb_event_length(event);
+
+ /*
+ * READ_ONCE() doesn't work on functions and we don't want the
+ * compiler doing any crazy optimizations with length.
+ */
+ barrier();
+
+ if ((iter->head + length) > commit || length > BUF_MAX_DATA_SIZE)
+ /* Writer corrupted the read? */
+ goto reset;
+
+ memcpy(iter->event, event, length);
+ /*
+ * If the page stamp is still the same after this rmb() then the
+ * event was safely copied without the writer entering the page.
+ */
+ smp_rmb();
+
+ /* Make sure the page didn't change since we read this */
+ if (iter->page_stamp != iter_head_page->page->time_stamp ||
+ commit > rb_page_commit(iter_head_page))
+ goto reset;
+
+ iter->next_event = iter->head + length;
+ return iter->event;
+ reset:
+ /* Reset to the beginning */
+ iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
+ iter->head = 0;
+ iter->next_event = 0;
+ return NULL;
}

/* Size is determined by what has been committed */
@@ -1962,6 +2008,7 @@ static void rb_inc_iter(struct ring_buffer_iter *iter)

iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
iter->head = 0;
+ iter->next_event = 0;
}

/*
@@ -3548,6 +3595,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
/* Iterator usage is expected to have record disabled */
iter->head_page = cpu_buffer->reader_page;
iter->head = cpu_buffer->reader_page->read;
+ iter->next_event = iter->head;

iter->cache_reader_page = iter->head_page;
iter->cache_read = cpu_buffer->read;
@@ -3625,7 +3673,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
return 0;

/* Still racy, as it may return a false positive, but that's OK */
- return ((iter->head_page == commit_page && iter->head == commit) ||
+ return ((iter->head_page == commit_page && iter->head >= commit) ||
(iter->head_page == reader && commit_page == head_page &&
head_page->read == commit &&
iter->head == rb_page_commit(cpu_buffer->reader_page)));
@@ -3853,15 +3901,22 @@ static void rb_advance_reader(struct ring_buffer_per_cpu *cpu_buffer)
static void rb_advance_iter(struct ring_buffer_iter *iter)
{
struct ring_buffer_per_cpu *cpu_buffer;
- struct ring_buffer_event *event;
- unsigned length;

cpu_buffer = iter->cpu_buffer;

+ /* If head == next_event then we need to jump to the next event */
+ if (iter->head == iter->next_event) {
+ /* If the event gets overwritten again, there's nothing to do */
+ if (rb_iter_head_event(iter) == NULL)
+ return;
+ }
+
+ iter->head = iter->next_event;
+
/*
* Check if we are at the end of the buffer.
*/
- if (iter->head >= rb_page_size(iter->head_page)) {
+ if (iter->next_event >= rb_page_size(iter->head_page)) {
/* discarded commits can make the page empty */
if (iter->head_page == cpu_buffer->commit_page)
return;
@@ -3869,27 +3924,7 @@ static void rb_advance_iter(struct ring_buffer_iter *iter)
return;
}

- event = rb_iter_head_event(iter);
-
- length = rb_event_length(event);
-
- /*
- * This should not be called to advance the header if we are
- * at the tail of the buffer.
- */
- if (RB_WARN_ON(cpu_buffer,
- (iter->head_page == cpu_buffer->commit_page) &&
- (iter->head + length > rb_commit_index(cpu_buffer))))
- return;
-
- rb_update_iter_read_stamp(iter, event);
-
- iter->head += length;
-
- /* check for end of page padding */
- if ((iter->head >= rb_page_size(iter->head_page)) &&
- (iter->head_page != cpu_buffer->commit_page))
- rb_inc_iter(iter);
+ rb_update_iter_read_stamp(iter, iter->event);
}

static int rb_lost_events(struct ring_buffer_per_cpu *cpu_buffer)
@@ -4017,6 +4052,8 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
}

event = rb_iter_head_event(iter);
+ if (!event)
+ goto again;

switch (event->type_len) {
case RINGBUF_TYPE_PADDING:
@@ -4233,10 +4270,16 @@ ring_buffer_read_prepare(struct trace_buffer *buffer, int cpu, gfp_t flags)
if (!cpumask_test_cpu(cpu, buffer->cpumask))
return NULL;

- iter = kmalloc(sizeof(*iter), flags);
+ iter = kzalloc(sizeof(*iter), flags);
if (!iter)
return NULL;

+ iter->event = kmalloc(BUF_MAX_DATA_SIZE, flags);
+ if (!iter->event) {
+ kfree(iter);
+ return NULL;
+ }
+
cpu_buffer = buffer->buffers[cpu];

iter->cpu_buffer = cpu_buffer;
@@ -4317,6 +4360,7 @@ ring_buffer_read_finish(struct ring_buffer_iter *iter)

atomic_dec(&cpu_buffer->record_disabled);
atomic_dec(&cpu_buffer->buffer->resize_disabled);
+ kfree(iter->event);
kfree(iter);
}
EXPORT_SYMBOL_GPL(ring_buffer_read_finish);
--
2.25.1

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