PROBLEM:a bug about pi-futex maybe let the program going to hang

[xby <scxby@163.com>]

hi, all.
 
  Maybe, there is a bug about pi-futex, it would let the program in user-space going to hang.
 
  We have a board: CPU is powerpc 8572, two core. after ran one month, the state of pi-futex in user-space got bad: mutex->__data.__lock is 0x8000023e, mutex->__data.__count is 0, mutex->__data.__owner is 0.
 
  then, I review file "kernel/funtex.c"(the version is linux 2.6.38), found a case:
 
  if there are 3 thread, named threadA, threadB, threadC。thread A hold mutexM, threadB and threadC is waiting mutexM. They run as fllow steps:
 
  1. threadB and threadC sleep at line 1984.
  2. threadB receive a signal, then it will be wake up.
  3. threadA unlock mutexM, and give mutexM to threadB. 
  4. threadB call fixup_owner, try to give mutex to threadC.
  5. at line 1580, threadB trigger a addr-fault, then goto handle_fault.
  6. at line 1617, threadB release spinlock, then handle fault.
  7. threadC got spinlock, and call fixup_owner, and got mutexM.
  8. threadC give mutexM to threadB.
  9. threadB re-got spinlock, it will found "pi_state->owner == oldowner" and retry to fixup.
  10. threadB give mutexM to threadC, that's a bad thing.
 
  we have wrote a program, this program can prove all above.