uretprobe occasionally fires twice

[Benjamin King <benjaminking@web.de>]

Hi,
 
I have placed a uprobe and a uretprobe on the same function in libc. When
perf record'ing a larger test where the function is called from multiple
threads, I can occasionally see the following sequence of events in a 'perf
script':
  
  1) uretprobe fires
  2) uprobe fires
  3) uretprobe fires again

More specifically, I am tracing libc's realloc. Since I trace the size of
the memory in question, I think that the uretprobe in 1) is called at
function entry.

This only happens occasionally and I unfortunately I could not reproduce it
with a small testcase yet.

'perf script' prints two warnings:
  * Processed 17216221 events and lost 2 chunks!
  * 114 out of order events recorded.

Could this explain the duplicated output? Are there other known circumstances
where a uretprobe could be unreliable in some way?

Cheers,
  Benjamin