[lttng-dev] [PATCH liburcu] Fix lifetime of rcu_barrier()'s completion structure

[Keir Fraser]

[Another attachment, I must sort out to integrate with git send-mail]

rcu_barrier() can return as soon as completion.barrier_count==0, which 
frees the completion struct along with the rest of its stack frame. But 
its call_rcu callbacks may yet try to read and write completion.futex 
via the wake_up function.

Fix this by calloc()ing the completion struct and implementing a 
reference count to determine when it is eventually free()d.

This also fixes bug #787, since calloc() initialises all fields of the 
structure to zero.

  Regards,
  Keir
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