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 If the reader page is swapped out of the buffer before an event is written to it, then the read_stamp may get an out of date timestamp, as the page timestamp is updated on the first commit to that page.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.
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| @@ -1887,12 +1887,6 @@ rb_event_index(struct ring_buffer_event *event) · 1886 unmodified lines | ||
| 1887 | 1887 | return (addr & ~PAGE_MASK) - BUF_PAGE_HDR_SIZE; |
| 1888 | 1888 | } |
| 1889 | 1889 | |
| 1890 | −static void rb_reset_reader_page(struct ring_buffer_per_cpu *cpu_buffer) | |
| 1891 | −{ | |
| 1892 | − cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp; | |
| 1893 | − cpu_buffer->reader_page->read = 0; | |
| 1894 | −} | |
| 1895 | − | |
| 1896 | 1890 | static void rb_inc_iter(struct ring_buffer_iter *iter) |
| 1897 | 1891 | { |
| 1898 | 1892 | struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer; |
| @@ -3626,7 +3620,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) · 1727 unmodified lines | ||
| 3626 | 3620 | |
| 3627 | 3621 | /* Finally update the reader page to the new head */ |
| 3628 | 3622 | cpu_buffer->reader_page = reader; |
| 3629 | − rb_reset_reader_page(cpu_buffer); | |
| 3623 | + cpu_buffer->reader_page->read = 0; | |
| 3630 | 3624 | |
| 3631 | 3625 | if (overwrite != cpu_buffer->last_overrun) { |
| 3632 | 3626 | cpu_buffer->lost_events = overwrite - cpu_buffer->last_overrun; |
| @@ -3636,6 +3630,10 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) · 3 unmodified lines | ||
| 3636 | 3630 | goto again; |
| 3637 | 3631 | |
| 3638 | 3632 | out: |
| 3633 | + /* Update the read_stamp on the first event */ | |
| 3634 | + if (reader && reader->read == 0) | |
| 3635 | + cpu_buffer->read_stamp = reader->page->time_stamp; | |
| 3636 | + | |
| 3639 | 3637 | arch_spin_unlock(&cpu_buffer->lock); |
| 3640 | 3638 | local_irq_restore(flags); |
| 3641 | 3639 | |