From 6039f028a9bbf7cb34ecfac31d5a9df68453221d Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (Red Hat)" Date: Mon, 23 Nov 2015 10:35:36 -0500 Subject: [PATCH] ring-buffer: Update read stamp with first real commit on page commit b81f472a208d3e2b4392faa6d17037a89442f4ce upstream. Do not update the read stamp after swapping out the reader page from the write buffer. If the reader page is swapped out of the buffer before an event is written to it, then the read_stamp may get an out of date timestamp, as the page timestamp is updated on the first commit to that page. rb_get_reader_page() only returns a page if it has an event on it, otherwise it will return NULL. At that point, check if the page being returned has events and has not been read yet. Then at that point update the read_stamp to match the time stamp of the reader page. Signed-off-by: Steven Rostedt Signed-off-by: Greg Kroah-Hartman --- kernel/trace/ring_buffer.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index ab21b8c66535..cb73c4e0741e 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -1948,12 +1948,6 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer) goto again; } -static void rb_reset_reader_page(struct ring_buffer_per_cpu *cpu_buffer) -{ - cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp; - cpu_buffer->reader_page->read = 0; -} - static void rb_inc_iter(struct ring_buffer_iter *iter) { struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer; @@ -3591,7 +3585,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) /* Finally update the reader page to the new head */ cpu_buffer->reader_page = reader; - rb_reset_reader_page(cpu_buffer); + cpu_buffer->reader_page->read = 0; if (overwrite != cpu_buffer->last_overrun) { cpu_buffer->lost_events = overwrite - cpu_buffer->last_overrun; @@ -3601,6 +3595,10 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) goto again; out: + /* Update the read_stamp on the first event */ + if (reader && reader->read == 0) + cpu_buffer->read_stamp = reader->page->time_stamp; + arch_spin_unlock(&cpu_buffer->lock); local_irq_restore(flags); -- 2.20.1