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

[Peter Zijlstra <peterz@infradead.org>]

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.