ring-buffer: Don't deactivate the ring buffer on failed iterator reads (3d2353de) · Commits · 戴 / test

If the function tracer is running and the trace file is read (which uses the ring buffer iterator), the iterator can get in sync with the writes, and caues it to fail to find a page with content it can read three times. This causes a warning and deactivation of the ring buffer code. Looking at the other cases of failure to get an event, it appears that there's a chance that the writer could cause them too. Since the iterator is a "best effort" to read the ring buffer if there's an active writer (the consumer reader is made for this case "see trace_pipe"), if it fails to get an event after three tries, simply give up and return NULL. Don't warn, nor disable the ring buffer on this failure. Link: https://lore.kernel.org/r/20200429090508.GG5770@shao2-debian Reported-by:

kernel test robot <lkp@intel.com> Fixes: ("ring-buffer: Do not die if rb_iter_peek() fails more than thrice") Tested-by:

Sven Schnelle <svens@linux.ibm.com> Signed-off-by:

Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

+7 −15

Original line number	Diff line number	Diff line
		@@ -4034,7 +4034,6 @@ rb_iter_peek(struct ring_buffer_iter iter, u64 ts)
		struct ring_buffer_per_cpu *cpu_buffer;
		struct ring_buffer_event *event;
		int nr_loops = 0;
		bool failed = false;

		if (ts)
		*ts = 0;
		@@ -4056,19 +4055,14 @@ rb_iter_peek(struct ring_buffer_iter iter, u64 ts)
		return NULL;

		/*
		* We repeat when a time extend is encountered or we hit
		* the end of the page. Since the time extend is always attached
		* to a data event, we should never loop more than three times.
		* Once for going to next page, once on time extend, and
		* finally once to get the event.
		* We should never hit the following condition more than thrice,
		* unless the buffer is very small, and there's a writer
		* that is causing the reader to fail getting an event.
		*/
		if (++nr_loops > 3) {
		RB_WARN_ON(cpu_buffer, !failed);
		* As the writer can mess with what the iterator is trying
		* to read, just give up if we fail to get an event after
		* three tries. The iterator is not as reliable when reading
		* the ring buffer with an active write as the consumer is.
		* Do not warn if the three failures is reached.
		*/
		if (++nr_loops > 3)
		return NULL;
		}

		if (rb_per_cpu_empty(cpu_buffer))
		return NULL;
		@@ -4079,10 +4073,8 @@ rb_iter_peek(struct ring_buffer_iter iter, u64 ts)
		}

		event = rb_iter_head_event(iter);
		if (!event) {
		failed = true;
		if (!event)
		goto again;
		}

		switch (event->type_len) {
		case RINGBUF_TYPE_PADDING:

Admin message