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ring-buffer: Do not die if rb_iter_peek() fails more than thrice 

As the iterator will be reading a live buffer, and if the event being read
is on a page that a writer crosses, it will fail and try again, the
condition in rb_iter_peek() that only allows a retry to happen three times
is no longer valid. Allow rb_iter_peek() to retry more than three times
without killing the ring buffer, but only if rb_iter_head_event() had failed
at least once.

Link: http://lkml.kernel.org/r/20200317213416.452888193@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware) 2020-03-17 17:32:28 -04:00
parent 785888c544
commit ff84c50cfb
1 changed files with 10 additions and 3 deletions
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13
kernel/trace/ring_buffer.c
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@ -4012,6 +4012,7 @@ 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;
@ -4038,10 +4039,14 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
* 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 never hit the following condition more than thrice).
* 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 (RB_WARN_ON(cpu_buffer, ++nr_loops > 3))
if (++nr_loops > 3) {
RB_WARN_ON(cpu_buffer, !failed);
return NULL;
}
if (rb_per_cpu_empty(cpu_buffer))
return NULL;
@ -4052,8 +4057,10 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
}
event = rb_iter_head_event(iter);
if (!event)
if (!event) {
failed = true;
goto again;
}
switch (event->type_len) {
case RINGBUF_TYPE_PADDING: