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

Peter Zijlstra
2011-03-28T08:26:50Z

On Mon, 2011-03-28 at 15:25 +0800, xby wrote:
> hi, all.

Works better if you also CC people who actually work on that code.

> 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.

It would have been ever so much more useful if you'd have included that.