[Bug ports/11291] potential deadlock in sem_*wait and sem_post for MIPS architectures

["mischa dot jonker at viragelogic dot com" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From mischa dot jonker at viragelogic dot com  2010-02-18 12:18 -------
I understand that adding 'volatile' does not change anything to the behaviour of the macro, and that the atomic 
instructions take care of other threads _while inside the atomic_compare_and_exchange_bool_acq macro_. The problem 
is, that it doesn't take care of concurrency in the instructions before that.

without volatile the compiler optimizes the code to:
  cur = isem->value;
  if (isem->value == SEM_VALUE_MAX)
   {
     __set_errno (EOVERFLOW);
     return -1;
   }
// if we get a context switch here, and someone else increases isem->value, 
// we get a deadlock, because cur contains isem->value+1, and
// atomic_compare_and_exchange_bool_acq can never succeed.
  do
   {
   }
 while (atomic_compare_and_exchange_bool_acq (&isem->value, cur + 1, cur));


So, while it doesn't alter the behaviour of atomic_compare_and_exchange_bool_acq, it does put the 'cur = isem->value' 
statement inside the while loop. However, changing the patch as proposed does this as well (although for different 
reasons: now the compiler is aware that the macro itself (instead of other threads) can change isem->value).


-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=11291

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