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The ring buffer reader page is used to swap a page from the writable ring buffer. If the writer happens to be on that page, it ends up on the reader page, but will simply move off of it, back into the writable ring buffer as writes are added. The time stamp passed back to the readers is stored in the cpu_buffer per CPU descriptor. This stamp is updated when a swap of the reader page takes place, and it reads the current stamp from the page taken from the writable ring buffer. Everytime a writer goes to a new page, it updates the time stamp of that page. The problem happens if a reader reads a page from an empty per CPU ring buffer. If the buffer is empty, the swap still takes place, placing the writer at the start of the reader page. If at a later time, a write happens, it updates the page's time stamp and continues. But the problem is that the read_stamp does not get updated, because the page was already swapped. The solution to this was to not swap the page if the ring buffer happens to be empty. This also removes the side effect that the writes on the reader page will not get updated because the writer never gets back on the reader page without a swap. That is, if a read happens on an empty buffer, but then no reads happen for a while. If a swap took place, and the writer were to start writing a lot of data (function tracer), it will start overflowing the ring buffer and overwrite the older data. But because the writer never goes back onto the reader page, the data left on the reader page never gets overwritten. This causes the reader to see really old data, followed by a jump to newer data. Link: http://lkml.kernel.org/r/1340060577-9112-1-git-send-email-dhsharp@google.com Google-Bug-Id: 6410455 Reported-by: David Sharp <dhsharp@google.com> tested-by: David Sharp <dhsharp@google.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org> |
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| .. | ||
| blktrace.c | ||
| ftrace.c | ||
| Kconfig | ||
| Makefile | ||
| power-traces.c | ||
| ring_buffer.c | ||
| ring_buffer_benchmark.c | ||
| rpm-traces.c | ||
| trace.c | ||
| trace.h | ||
| trace_branch.c | ||
| trace_clock.c | ||
| trace_entries.h | ||
| trace_event_perf.c | ||
| trace_events.c | ||
| trace_events_filter.c | ||
| trace_events_filter_test.h | ||
| trace_export.c | ||
| trace_functions.c | ||
| trace_functions_graph.c | ||
| trace_irqsoff.c | ||
| trace_kdb.c | ||
| trace_kprobe.c | ||
| trace_mmiotrace.c | ||
| trace_nop.c | ||
| trace_output.c | ||
| trace_output.h | ||
| trace_printk.c | ||
| trace_probe.c | ||
| trace_probe.h | ||
| trace_sched_switch.c | ||
| trace_sched_wakeup.c | ||
| trace_selftest.c | ||
| trace_selftest_dynamic.c | ||
| trace_stack.c | ||
| trace_stat.c | ||
| trace_stat.h | ||
| trace_syscalls.c | ||
| trace_uprobe.c | ||